A student I am working with is designing a four valve head for a 3.5 Hp Briggs engine. We are planning to make the head in three parts. The parting lines join in the combustion chamber. How could these parts be joined and still have compression? Does anyone have any good ideas?
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DOH Cam Head Design 3
- Thread starter MRBB
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Considering the bottom end strength and the limited heat rejection charistics of a Briggs, this appears to be a design effort in futility---be that as it may, whatever floats your boat.
The heat rejection problem will limit your ability to use a very high CR (even with methonol based fuels) , so it might be possible to make the joints a machined (properly lapped) surface. I am having a little difficulty in visualizing your project. Perhaps a bit more detail? Why is it necessary to cast THREE pieces in order to machine the ports? (I can see TWO, but not THREE) WHY do you find it necessary to machine the ports in the first place? ARE you casting this head or trying to fab it from billet? I have been privy to the casting of several DOHC heads with little difficulty in port design. Even a pushrod Briggs about 40 years ago (a wasted effort for above reasons)! Perhaps you can fill in some of the blanks, yes?
Rod
The heat rejection problem will limit your ability to use a very high CR (even with methonol based fuels) , so it might be possible to make the joints a machined (properly lapped) surface. I am having a little difficulty in visualizing your project. Perhaps a bit more detail? Why is it necessary to cast THREE pieces in order to machine the ports? (I can see TWO, but not THREE) WHY do you find it necessary to machine the ports in the first place? ARE you casting this head or trying to fab it from billet? I have been privy to the casting of several DOHC heads with little difficulty in port design. Even a pushrod Briggs about 40 years ago (a wasted effort for above reasons)! Perhaps you can fill in some of the blanks, yes?
Rod
- Thread starter
- #5
This idea of a 4 valve head came from a SAE paper about making a 50cc 4 cycle engine more fuel efficient. The student I am helping, is required by rules of a Super Mileage challenge to use a 90000 series Briggs engine, 3.5HP. We don't understand the heat rejection problems you are talking about. This project is not to make more power, but rather decrease fuel consumption. We are not sure how much cr we can get away with, probably 9:1 max.??? We are not planning to cast this head, but rather mill the 4 ports, fasten the three pieces together and then mill the combustion chamber, valve seat holes, etc. We are in a bit over our heads, but are willing to learn. Suggestions?
OK---The 9:1 sounds about right. The overheat stems from trying to run 13/14:1 CR at 6000+ RPM. Since that is not your goal, heat will probably not enter into the equation. As long as you don't run too high in the rev range and, as long as you keep the CR down it will perform admirably (the connecting rod is the weak link).
Now the billitt head---I have never seen anyone try that so I am not qualified to comment aside from the fact that it seems the hard way to go. Casting a solid cylinder head with no cooling ducting seems symple enough. At least you could design ports in any location that will clear the head studs and machine combustion chamber or valve seats as needed. The 3.5 is a very inexpensive engine and I have never worked on a Briggs that small, mostly 5 hp models for go carts and mini bikes back in the '60's.
Sorry I can't be of more help. Never had any thoughts of making an econo engine. Fuel mileage in my type of racing is of limited concern. Good luck with your project.
Don't worry about being in over you heads. That's SOP around here! LOL "Nothing ventured, nothing gained".
Rod
Now the billitt head---I have never seen anyone try that so I am not qualified to comment aside from the fact that it seems the hard way to go. Casting a solid cylinder head with no cooling ducting seems symple enough. At least you could design ports in any location that will clear the head studs and machine combustion chamber or valve seats as needed. The 3.5 is a very inexpensive engine and I have never worked on a Briggs that small, mostly 5 hp models for go carts and mini bikes back in the '60's.
Sorry I can't be of more help. Never had any thoughts of making an econo engine. Fuel mileage in my type of racing is of limited concern. Good luck with your project.
Don't worry about being in over you heads. That's SOP around here! LOL "Nothing ventured, nothing gained".
Rod
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2
- #7
The following are my beliefs only, based on more theoretical knowledge than first-hand factual experience.
1) There is no one magic bullet to economy. For more, see the tornado swirling device thread of a month ago.
2) Economy is about combustion efficiency and the transference of this force to do work.
3) Sometimes, an engine built for speed and one for economy share identical goals and make nearly the same power.
Now, since you probably took some physics, you know how a system is comprised of many parts. The engine will thus be part of a system, that is unless we are simply going to run it on a dyno, in which case it will be the only system.
If it will go into a vehicle of some sort, you will need to integrate it as best possible (with gear ratios for economy, etc.). If it isn't, you will not worry about it.
So, what makes an engine efficient? These are my ideas..
As I stated above, it all hinges on combustion efficiency. The faster the charge can be made to burn, the more potential exists for optimum power production from a given unit of fuel.
Why is burn rate important? Timing! Anyone who has timed an engine knows you must set the ignition to fire sometime prior to TDC. This is to get the fuel/air mix ignited prior to the power stroke, so as to generate the greatest cylinder pressure during that time. However, it does not take a very astute person to realize that power expended on the upward (compression) stroke of the piston, is work in the wrong direction. Thus it could be termed, "negative work" which is being done prior to the power stroke proper.
It should now be apparent that we would like to limit the amount of "negative work" being produced. Additionally, it would be nice to be able to use some of the work which was previously being expended on the compression stroke for power production on the power stroke. The only way to do this, is to increase the rate of combustion. A faster burning charge will not have to be ignited as early in the compression stroke to reach the same pressure levels on the power stroke, and thus the engine will make more power for a given quantity of fuel/air mix and speed.
So how do we increase the rate of combustion? Let's start with airflow 'quality' (as it's been termed).
When most think of airflow, they think of quantity, usually thinking the more the merrier. This is basically OK for an engine built for speed, but we are not, so assuming sufficient quantity, quality will be of priority.
At some point in the induction process, fuel will be introduced to the incoming stream of air. This fuel must be administered in as atomized and homogenous a form as possible. This level of atomization MUST be maintained up until, and through the time of combustion. This is because fuel which separates from the air, or otherwise clumps together, will not burn as quickly as fuel which is finely atomized. Put simply, liquid fuel will not burn! Throw a match in a bucket of gasoline and see what happens some time.
To keep fuel from separating, you must pay careful consideration to the ducting design. As air flows through a port which turns, the air will "hug" the inside radius of the turns. If you take a string and pull it through a port, the line you see represented will be a very close approximation to the way the air is flowing. When the air tries to do the above, you will have a distribution of pressure throughout the port. Thus, if you were to slice a cross section of the port through a turn for instance, you would find that the air pressure along the short path would be substantially lower than along the "long side" . This has a tendency to create shearing in the flow as the velocity will also differ with the pressure. All of the above causes the fuel to be drawn out of the flow stream.
To solve the above problem the ducting will have to take on D or trapezoidal shaping, with the short paths being widened and the long sides "shrunk" to help balance the pressures.
There is another problem with air/fuel mix which you should know about also. Fuel is obviously heavier than the air which is carrying it. If you encounter a turn, the fuel will try to go straight by means of it's inertia, while the air will seek the lower pressures encountered on the other side of the bend. It can help to increase the size of a duct through a turn to counter-act this effect. The area will need to be decreased again on the other side of the turn.
I will also add that the area just before the valve seat should always be smaller than the rest of the port and this area should be between about 84-92% of the valve diameter. I'll let you figure out the rest.
The last consideration is port wall lengths. The larger the difference in length between each portion of the port, the bigger the difference in surface "friction" imposed and the greater the chance of sheering and subsequent fuel separation. Thus the port floor, roof, and walls should measure the same in all cases. You will find this difficult to do, but do not forget this point, as it makes design and modification easier to understand, do the best you can.
The last big bugaboo prior to the combustion chamber are the seats. All I can say is that the seat is extremely critical to not only airflow quantity, but quality as well. Keep the length differences noted above in mind and carry that ideal all the way to the seat itself. The long side of the port should have virtually no bottom angle (nearly straight from the guide, shortest wall length) with the short side as generous a radius as possible right into the seat (longest wall length). Don't go hog-wild on a top angle, something like a .010" 15 to 30 degree (or less) is best in nearly all cases. This is something you could take a long time developing, so just keep the top angle minimal.
We now find ourselves in the combustion chamber and I will say that two things are important. One is turbulence, the other is surface area to volume at TDC.
Turbulence should always be oriented (not random) and should always be in the direction of swirl as opposed to tumble, which is a random form (in my opinion). You want the swirl to make about two loops of the combustion chamber, which you can figure out by using an RPM reading swirl meter and about two years of time spent making calculations to relate flowbench results to actual running conditions. Suffice it is to say, you won't spend this time, so think about the following. Since you'll be making your own cam and follower design, you now have the ability to very the timing between both inlet valves so as to start one opening prior to the other. This can be used to introduce a bias to the flow, and thus swirl. Also consider lift for this. I'd make a guess of 5 degrees timing difference and about .030" lift difference. Try it and see what it likes if you have the time to do it. If you don't, just go with everything else I've said and forget about it.
Combustion chamber design will also influence the flow, walls placed near the valve can cause the flow to do all kinds of screwy stuff, so try to keep everything at an equal distance if you can.
Lastly, you need to consider how the charge will be confined at TDC. You would like the charge to be "pushed" to the exhaust side where it will be heated to a point where it will burn faster. The spark plug should also be in the vacinity of the exhaust side for proper combustion. This is very important for the mixture to burn quickly and efficiently. You can achieve this effect by placing a pent-roof dome on the piston that will 'quench-off' the intake side of the combustion chamber, and thus force the mixture to the exhaust side. Since you will be designing the head, you can put the spark plug (or better yet, plugs) on the exhaust side, and the heat, combined with their placement will work for you.
Lastly, you need to get the exhaust out quickly. Exhaust residue in the chamber will slow the rate of combustion, which is not what we are after! Exhaust seats should be full radius, but with a 45 degree seat. Try a steep back angle on the valve (35 to 40 degrees works best usually). All the rules of inlet port work apply to the exhaust, but you might consider making the port no larger than the exhaust valve throat, or possibly even smaller for best velocity. Obviously wave tuning is important, but I'm somewhat 'green' regarding that.
Lastly, smooth doesn't mean anything to the airflow. I would texture the inlet ports with no less than 60 grit sandpaper on a split rod, scratches perpendicular to the direction of airflow. 80 grit is ideal for the rest of it, chamber and exhaust ports respectively.
I'd look at the modern crop of Honda engines for a good idea of how things are evolving. Try to keep the valve angle shallow for the minimum chamber depth and the lowest piston dome that will achieve the above mentioned ideals. You may have to run something like 12:1 or higher compression to effectively quench off the inlet side, and I wouldn't worry too much about doing it. You may also find you need relatively short intake cam timing to keep the inlet valve reliefs as small as possible, don't worry about it. You care only about combustion efficiency, so beyond airflow quality, quantity can take a back seat for awhile.
Good luck, have fun.
Sean
1) There is no one magic bullet to economy. For more, see the tornado swirling device thread of a month ago.
2) Economy is about combustion efficiency and the transference of this force to do work.
3) Sometimes, an engine built for speed and one for economy share identical goals and make nearly the same power.
Now, since you probably took some physics, you know how a system is comprised of many parts. The engine will thus be part of a system, that is unless we are simply going to run it on a dyno, in which case it will be the only system.
If it will go into a vehicle of some sort, you will need to integrate it as best possible (with gear ratios for economy, etc.). If it isn't, you will not worry about it.
So, what makes an engine efficient? These are my ideas..
As I stated above, it all hinges on combustion efficiency. The faster the charge can be made to burn, the more potential exists for optimum power production from a given unit of fuel.
Why is burn rate important? Timing! Anyone who has timed an engine knows you must set the ignition to fire sometime prior to TDC. This is to get the fuel/air mix ignited prior to the power stroke, so as to generate the greatest cylinder pressure during that time. However, it does not take a very astute person to realize that power expended on the upward (compression) stroke of the piston, is work in the wrong direction. Thus it could be termed, "negative work" which is being done prior to the power stroke proper.
It should now be apparent that we would like to limit the amount of "negative work" being produced. Additionally, it would be nice to be able to use some of the work which was previously being expended on the compression stroke for power production on the power stroke. The only way to do this, is to increase the rate of combustion. A faster burning charge will not have to be ignited as early in the compression stroke to reach the same pressure levels on the power stroke, and thus the engine will make more power for a given quantity of fuel/air mix and speed.
So how do we increase the rate of combustion? Let's start with airflow 'quality' (as it's been termed).
When most think of airflow, they think of quantity, usually thinking the more the merrier. This is basically OK for an engine built for speed, but we are not, so assuming sufficient quantity, quality will be of priority.
At some point in the induction process, fuel will be introduced to the incoming stream of air. This fuel must be administered in as atomized and homogenous a form as possible. This level of atomization MUST be maintained up until, and through the time of combustion. This is because fuel which separates from the air, or otherwise clumps together, will not burn as quickly as fuel which is finely atomized. Put simply, liquid fuel will not burn! Throw a match in a bucket of gasoline and see what happens some time.
To keep fuel from separating, you must pay careful consideration to the ducting design. As air flows through a port which turns, the air will "hug" the inside radius of the turns. If you take a string and pull it through a port, the line you see represented will be a very close approximation to the way the air is flowing. When the air tries to do the above, you will have a distribution of pressure throughout the port. Thus, if you were to slice a cross section of the port through a turn for instance, you would find that the air pressure along the short path would be substantially lower than along the "long side" . This has a tendency to create shearing in the flow as the velocity will also differ with the pressure. All of the above causes the fuel to be drawn out of the flow stream.
To solve the above problem the ducting will have to take on D or trapezoidal shaping, with the short paths being widened and the long sides "shrunk" to help balance the pressures.
There is another problem with air/fuel mix which you should know about also. Fuel is obviously heavier than the air which is carrying it. If you encounter a turn, the fuel will try to go straight by means of it's inertia, while the air will seek the lower pressures encountered on the other side of the bend. It can help to increase the size of a duct through a turn to counter-act this effect. The area will need to be decreased again on the other side of the turn.
I will also add that the area just before the valve seat should always be smaller than the rest of the port and this area should be between about 84-92% of the valve diameter. I'll let you figure out the rest.
The last consideration is port wall lengths. The larger the difference in length between each portion of the port, the bigger the difference in surface "friction" imposed and the greater the chance of sheering and subsequent fuel separation. Thus the port floor, roof, and walls should measure the same in all cases. You will find this difficult to do, but do not forget this point, as it makes design and modification easier to understand, do the best you can.
The last big bugaboo prior to the combustion chamber are the seats. All I can say is that the seat is extremely critical to not only airflow quantity, but quality as well. Keep the length differences noted above in mind and carry that ideal all the way to the seat itself. The long side of the port should have virtually no bottom angle (nearly straight from the guide, shortest wall length) with the short side as generous a radius as possible right into the seat (longest wall length). Don't go hog-wild on a top angle, something like a .010" 15 to 30 degree (or less) is best in nearly all cases. This is something you could take a long time developing, so just keep the top angle minimal.
We now find ourselves in the combustion chamber and I will say that two things are important. One is turbulence, the other is surface area to volume at TDC.
Turbulence should always be oriented (not random) and should always be in the direction of swirl as opposed to tumble, which is a random form (in my opinion). You want the swirl to make about two loops of the combustion chamber, which you can figure out by using an RPM reading swirl meter and about two years of time spent making calculations to relate flowbench results to actual running conditions. Suffice it is to say, you won't spend this time, so think about the following. Since you'll be making your own cam and follower design, you now have the ability to very the timing between both inlet valves so as to start one opening prior to the other. This can be used to introduce a bias to the flow, and thus swirl. Also consider lift for this. I'd make a guess of 5 degrees timing difference and about .030" lift difference. Try it and see what it likes if you have the time to do it. If you don't, just go with everything else I've said and forget about it.
Combustion chamber design will also influence the flow, walls placed near the valve can cause the flow to do all kinds of screwy stuff, so try to keep everything at an equal distance if you can.
Lastly, you need to consider how the charge will be confined at TDC. You would like the charge to be "pushed" to the exhaust side where it will be heated to a point where it will burn faster. The spark plug should also be in the vacinity of the exhaust side for proper combustion. This is very important for the mixture to burn quickly and efficiently. You can achieve this effect by placing a pent-roof dome on the piston that will 'quench-off' the intake side of the combustion chamber, and thus force the mixture to the exhaust side. Since you will be designing the head, you can put the spark plug (or better yet, plugs) on the exhaust side, and the heat, combined with their placement will work for you.
Lastly, you need to get the exhaust out quickly. Exhaust residue in the chamber will slow the rate of combustion, which is not what we are after! Exhaust seats should be full radius, but with a 45 degree seat. Try a steep back angle on the valve (35 to 40 degrees works best usually). All the rules of inlet port work apply to the exhaust, but you might consider making the port no larger than the exhaust valve throat, or possibly even smaller for best velocity. Obviously wave tuning is important, but I'm somewhat 'green' regarding that.
Lastly, smooth doesn't mean anything to the airflow. I would texture the inlet ports with no less than 60 grit sandpaper on a split rod, scratches perpendicular to the direction of airflow. 80 grit is ideal for the rest of it, chamber and exhaust ports respectively.
I'd look at the modern crop of Honda engines for a good idea of how things are evolving. Try to keep the valve angle shallow for the minimum chamber depth and the lowest piston dome that will achieve the above mentioned ideals. You may have to run something like 12:1 or higher compression to effectively quench off the inlet side, and I wouldn't worry too much about doing it. You may also find you need relatively short intake cam timing to keep the inlet valve reliefs as small as possible, don't worry about it. You care only about combustion efficiency, so beyond airflow quality, quantity can take a back seat for awhile.
Good luck, have fun.
Sean
- Thread starter
- #8
another thing to think about if you are designing for economy is having the effective compression ratio of the intake stroke different from the effective expansion ratio of the exahust stroke the way to do this is have the intake valves stay open part way up the compression stroke, so the volume you are compressing is less. this would make the combustion chamber volume smaller as well.so instead of having a 9 to 1 compression ratio and expansion ratio you could have 9 to 1 compression and 12 to one expansion. this will reduce hp per cubic in but will lower bsfc. resulting in greater fuel economy (all other things equal)
also aluminum heads allow a higher cr, but also lose a bit more power to heat than iron.
I would suggest an aluminum head and a solid copper head gasket as this would help cool the block as much as posible and allow the cr to be as high as it can be given your fuel/engine combo.
you may look into atkinson cycle what I described is a variation on this concept similar to what some of the current production hev's are using.
also aluminum heads allow a higher cr, but also lose a bit more power to heat than iron.
I would suggest an aluminum head and a solid copper head gasket as this would help cool the block as much as posible and allow the cr to be as high as it can be given your fuel/engine combo.
you may look into atkinson cycle what I described is a variation on this concept similar to what some of the current production hev's are using.
- Thread starter
- #10
We have a heat problem, we can't keep the engine hot enough. A good high mileage car will burn its engine for only about 3 seconds and coast for almost 60 seconds with the engine off. We have considered wrapping the engine block with aluminum tubing, a storage tank, radiator, coolant and a pump, with the whole thing insulated and thermostatically controlled. Then again, it's one more system to build and control and it adds weight. Your idea of holding the intake valves open longer to decrease intake cr sounds good. We have calculated cr using the volume the piston sweeps vs the volume in the head. Is this correct, and if so, how do we take into consideration for different valve opening and closings times or angles?
- Thread starter
- #11
Sean suggested, I think, using more than one spark plug. This idea was suggested to the guys working on this project, and they were advised by an automotive instructor, not to go that route unless they had a method of delaying the second plug. The instructor was worried about starting the fire from to locations and causing shock waves. Any ideas about this problem?
As to your origional post, yes, you can make your head out of multiple pieces of aluminum, and then furnace braze them together. This is an expensive process with a learning curve. I would not suggest this method.
For a one-piece prototype, I would drill the ports and finish them with a die grinder. With the proper tools, an experienced porter could finish the ports in about 5 hours. For a first-timer with make-do tools, I would expect 20-40 hours.
As to 2 spark plugs, you are going to run out of room very quickly. I see no advantage, and a lot of work given your small bore size. Also, how are you going to fit another magneto, add a second flywheel?
As for keeping the engine warm while it's off, why not wrap it with insulation? There are some good header wraps that will maintain the heat with minimial fuss or work.
For a one-piece prototype, I would drill the ports and finish them with a die grinder. With the proper tools, an experienced porter could finish the ports in about 5 hours. For a first-timer with make-do tools, I would expect 20-40 hours.
As to 2 spark plugs, you are going to run out of room very quickly. I see no advantage, and a lot of work given your small bore size. Also, how are you going to fit another magneto, add a second flywheel?
As for keeping the engine warm while it's off, why not wrap it with insulation? There are some good header wraps that will maintain the heat with minimial fuss or work.
You won't have to worry about "shock waves!" There are numerous examples of engines running two plugs per cylinder and I have never heard of them having such a problem. Nearly all light aircraft engines use two plugs per cylinder and dual magnetos. When you switch between mags on a run-up you can definitely tell when only one is firing by the way the engine runs (or doesn't).
Top-fuel dragsters and funny-cars do the exact same thing.
I think a clever person could adapt two mags to the same flywheel with about 0 difficulty.
CR = Swept volume + clearance volume / clearance volume.
I would probably not let the mixture be pumped back out as suggested. Were I to explore the idea of a differing CR/ER concept, I would simply limit intake timing instead of making it too long. I believe this method would be better for mixture integrity which is important for a fast burn rate. I'm also assuming a narrow RPM range for all of this.
Sean
Top-fuel dragsters and funny-cars do the exact same thing.
I think a clever person could adapt two mags to the same flywheel with about 0 difficulty.
CR = Swept volume + clearance volume / clearance volume.
I would probably not let the mixture be pumped back out as suggested. Were I to explore the idea of a differing CR/ER concept, I would simply limit intake timing instead of making it too long. I believe this method would be better for mixture integrity which is important for a fast burn rate. I'm also assuming a narrow RPM range for all of this.
Sean
to take into consideration the differant valve events isn't that hard but isn't totaly precice. basicly compute your cr based on the swept volume from when the intake valve closes as opposed to the bottom of the cylnder, the thing to remenber is that the intake valve is usualy open for a small period anyway so you have to take that into consideration. the way arround this mathmaticly is to do your computations based on compression psi as opposed to ratio, but that isn't how it is historicly done so that isn't a perfect solution either.
SWB suggested not having the intake open as long as opposed to leaving it open extra, that should work also, however the two methods must produce differant charicteristics. the reason I suggested leaving the valve open longer is because that is the solution that manufactureers seem to be using from the information I have been able to get, so there may be a reason to do it that way. or not?
SWB suggested not having the intake open as long as opposed to leaving it open extra, that should work also, however the two methods must produce differant charicteristics. the reason I suggested leaving the valve open longer is because that is the solution that manufactureers seem to be using from the information I have been able to get, so there may be a reason to do it that way. or not?
patprimmer
New member
late inlet valve closeing causes reversion, as the piston comeing up on compression pushes air back out the valve into the port.
On a fuel injected engine, this is a slight problem, but on a carburetored engine, it is a disaster re fuel efficiency.
As the air flows back and forward through the venturi, it draws fuel on each pass, considerably richening the mixture.
As engine speed increases, the inertia of the air in the port overcomes the air being pushed out by the piston, overcoming reversion and thus the air only goes through the venturi once.
It is impossible to jet the carby correctly for both circumstances.
If you have fuel injection, you can metre the fuel accurately, but you still get a reduction in mixture quality as the air and fuel seperate on the direction change, due to their different inertia.
This will not be a problem if the engine can be kept in it's happy range, but this range will be narrow, and the port cross sectional area and overall length of the inlet tract will need to be tuned to be optimised for the desired rev range, as will the valve timeing, so that all components are happiest at the same speed.
Thermal barrier type ceramic coating of the combustion chamber, valves and piston will also help thermal efficency.
On a fuel injected engine, this is a slight problem, but on a carburetored engine, it is a disaster re fuel efficiency.
As the air flows back and forward through the venturi, it draws fuel on each pass, considerably richening the mixture.
As engine speed increases, the inertia of the air in the port overcomes the air being pushed out by the piston, overcoming reversion and thus the air only goes through the venturi once.
It is impossible to jet the carby correctly for both circumstances.
If you have fuel injection, you can metre the fuel accurately, but you still get a reduction in mixture quality as the air and fuel seperate on the direction change, due to their different inertia.
This will not be a problem if the engine can be kept in it's happy range, but this range will be narrow, and the port cross sectional area and overall length of the inlet tract will need to be tuned to be optimised for the desired rev range, as will the valve timeing, so that all components are happiest at the same speed.
Thermal barrier type ceramic coating of the combustion chamber, valves and piston will also help thermal efficency.
If you keep it open, especially on a one-lunger, wouldn't you run into problems with reversion back out the carb? On a multi-cylinder engine, its reasonable that another cylinder will be opening up, but not on a one-lunger. Closing the valve early seems like it would cause a little more loss in power by trying to draw in a vacuum ('compression' braking) but you wouldn't have the reversion problems.
Seems to me that designing a smaller motor to put out more power (as alluded to above) would be the way to go. A 'regular' cycle would, while not giving that cool factor of an alternative cycle, be much easier and let you spend your time on other things (port dynamics, twin plugs (even Ford made a twin-plug head for the 2.3L), tiny bearings, etc).
-=Whittey=-
Seems to me that designing a smaller motor to put out more power (as alluded to above) would be the way to go. A 'regular' cycle would, while not giving that cool factor of an alternative cycle, be much easier and let you spend your time on other things (port dynamics, twin plugs (even Ford made a twin-plug head for the 2.3L), tiny bearings, etc).
-=Whittey=-
patprimmer
New member
If you are going to make a cylinder head from 3 aluminium billits, why not just weld them together. In the 70's and 80's I used to hot rod air cooled Volkswagens.
When we needed port sizes that went outside the original port wall thickness, we just built up the outside of the port runner with aluminium welding rod, then realy went for it with the die grinder, with no fear of breaking through.
This was cast aluminium which should be harder to weld than billit. These heads ran over 100,000 miles in a street car making more than 5 times the engines stock HP, so durability of the welds was obviously not a problem
When we needed port sizes that went outside the original port wall thickness, we just built up the outside of the port runner with aluminium welding rod, then realy went for it with the die grinder, with no fear of breaking through.
This was cast aluminium which should be harder to weld than billit. These heads ran over 100,000 miles in a street car making more than 5 times the engines stock HP, so durability of the welds was obviously not a problem
Hello Sean/SWB.
great extensive post- I agree with some of it, I'd like to post some of my slants on it though, if that's OK.
First off I agree that economy is ALOT to do with combustion efficiency but engine friction should not be over looked. this is obvious I'm sure.
Fortunately , I've also found that engine often built for performance share similar fundamental ingriedients with economy engines.
You talked of liquid fuel and fuel which had seperated. This can also happen in "flame front quench zones" I have found, which in turn can arise when someone gets the squich clearance in an engine wrong- you want to be 0.9-1mm piston crown squich area impinging on the head, but not bigger- unless you've gone to the point where it's no longer intended as a squich zone.
To solve potential problems of fuel falling out of the flow stream I often look to finer atomisation injectors, but perhaps this is not applicable depending on the fuel system being used.
Regarding shaped ports:
I usually used D sectioned ports whem I’m trying to bias the flow over the top of the valve or trying to area compensate-for say- a valve guide shorud perhaps-otherwise I stick to circular ports for a two valve applications for outright flow.
In terms of the fuel inertia issues -with the fuel wanting to perhaps trace a wider path in the air flow, again I find finer atomisation injectors pays dividends, paying close attension to injector targeting, and positioning as close to the inlet valves as is practical.
Good luck in relating swirl and tumble meter readings from flow benchs to real engines. Flow benches are steady state devices, USUALLY with limited pressure drop abilities (10-20 inches of water) where as resserch has shown real engines can reach up to 70 inches of water pressure drop and are pulsating transient flow devices. As a result flow losses in REAL engines can be up to 6 times worse then shown on steady state tests, but this is special worst case.
Big Oem companies often have seeded water flow rigs in mock engines so that flow in an actual engine can me visuaised and related to test data.
It’s been found that domes in pistons increase the surface to volume ratio, adversely ( this was common practice in the past I know!)- it will also tend to shroud and shield the propagation of the flame front. I would defintaley agree with placing the plugs as close to the hot exhaust valves as is practical-in order to be closer to the end gas.
I’m not sure I agree with quenching off the inlet side of the chamber, as this could lead to unburned hydrocarbons…and thus higher BSFC.
Regarding EGR or exhaust gas residuals:
Infact Where as EGR or exhaust gas residual was used simple solely to reduce Nox emissions in the old days, it is used nowadays to INCREASE fuel economy. This is done at part load, by forcing the engine to run at a wide more open throttle position for a given torque thus reducing pumping losses. Usually at full load WOT the tendency for cylinder residuals to lurk is diminished in any case.
And finally, Honda engines I’ve seen are competent, but I would never rate them above BMWs. I was very dissappointed when I saw the ports of the S2000. Very very big, even when you consider it’s mammoth rev range, and not a very fluid shape. Definitely a tumble port though- but only machined in the throat area.
great extensive post- I agree with some of it, I'd like to post some of my slants on it though, if that's OK.
First off I agree that economy is ALOT to do with combustion efficiency but engine friction should not be over looked. this is obvious I'm sure.
Fortunately , I've also found that engine often built for performance share similar fundamental ingriedients with economy engines.
You talked of liquid fuel and fuel which had seperated. This can also happen in "flame front quench zones" I have found, which in turn can arise when someone gets the squich clearance in an engine wrong- you want to be 0.9-1mm piston crown squich area impinging on the head, but not bigger- unless you've gone to the point where it's no longer intended as a squich zone.
To solve potential problems of fuel falling out of the flow stream I often look to finer atomisation injectors, but perhaps this is not applicable depending on the fuel system being used.
Regarding shaped ports:
I usually used D sectioned ports whem I’m trying to bias the flow over the top of the valve or trying to area compensate-for say- a valve guide shorud perhaps-otherwise I stick to circular ports for a two valve applications for outright flow.
In terms of the fuel inertia issues -with the fuel wanting to perhaps trace a wider path in the air flow, again I find finer atomisation injectors pays dividends, paying close attension to injector targeting, and positioning as close to the inlet valves as is practical.
Good luck in relating swirl and tumble meter readings from flow benchs to real engines. Flow benches are steady state devices, USUALLY with limited pressure drop abilities (10-20 inches of water) where as resserch has shown real engines can reach up to 70 inches of water pressure drop and are pulsating transient flow devices. As a result flow losses in REAL engines can be up to 6 times worse then shown on steady state tests, but this is special worst case.
Big Oem companies often have seeded water flow rigs in mock engines so that flow in an actual engine can me visuaised and related to test data.
It’s been found that domes in pistons increase the surface to volume ratio, adversely ( this was common practice in the past I know!)- it will also tend to shroud and shield the propagation of the flame front. I would defintaley agree with placing the plugs as close to the hot exhaust valves as is practical-in order to be closer to the end gas.
I’m not sure I agree with quenching off the inlet side of the chamber, as this could lead to unburned hydrocarbons…and thus higher BSFC.
Regarding EGR or exhaust gas residuals:
Infact Where as EGR or exhaust gas residual was used simple solely to reduce Nox emissions in the old days, it is used nowadays to INCREASE fuel economy. This is done at part load, by forcing the engine to run at a wide more open throttle position for a given torque thus reducing pumping losses. Usually at full load WOT the tendency for cylinder residuals to lurk is diminished in any case.
And finally, Honda engines I’ve seen are competent, but I would never rate them above BMWs. I was very dissappointed when I saw the ports of the S2000. Very very big, even when you consider it’s mammoth rev range, and not a very fluid shape. Definitely a tumble port though- but only machined in the throat area.
Marquis,
I think it's SAAB that's doing the exhaust gas reversion technology, correct? Have other OEM's jumped on the bandwagon to your knowledge?
While not all of my research has been along the following lines concerning the quenching off of the inlet side, some of it has.
One very well known example of an engine utilizing the above techniques of combustion chamber "biasing" is the V-12 (double six) Jaguar previously used in the X-JS. I suspect you would have privileged knowledge of this engine extending well beyond mine. None the less, I believe the concept was instigated at the time by Michael May, allowing Jaguar to meet emissions and fuel economy standards previously not obtainable from the Heron type (combustion-cell piston) layout they started with. If I remember correctly the CR of that engine was/is over 11:1 and it runs comfortably on premium fuel.
I would be interested to know any more you may have to offer on the situation.
For those at home, the cylinder head was flat on the inlet side with the exhaust valve and spark plug housed in a "trench" making up the remainder of the "chamber." I recall the piston was a flat top. The heads were a single overhead cam type.
I would like to add that I've read quite a lot lately about the merits of a very shallow chamber with either a flat top or "mirrored dish" type piston, allowing the minimum surface area to volume ratio. This all sounds real good, but think of the following, which has apparently not been considered by the champions of the following ideals.
Taking the idea to the extreme, we find a chamber that is very wide and very thin with the spark plug in the center. This I'm told by varying experts would be "ideal!" What we have now is analogous to a flat sheet of paper! Try burning a flat sheet of paper and see how long it takes to get to the edges.
What I propose is instead confining the mixture into a volume of likely the same (but preferably less) area, but instead, we're going to shorten the distance from any given point relative to the spark plug by "shutting off" the inlet side (analogous to crumpling the paper). Indeed when the piston is down the bore, the surface area of it is greater than when at TDC. But when the piston is at TDC, the surface area of the "biased" piston dome would have to be less.
I proposed this technique to the originator of the post, as it's pretty easy to do with the pent-roof chamber design by simply angling the dome to closely coincide with chamber on the inlet side, while making the "slope" of the dome considerably steeper on the exhaust. I agree with Marquis about the squish clearance. You do not want to trap mixture somewhere on the inlet side, it must all be forced to the exhaust side or the design is a no-go.
Sean
I think it's SAAB that's doing the exhaust gas reversion technology, correct? Have other OEM's jumped on the bandwagon to your knowledge?
While not all of my research has been along the following lines concerning the quenching off of the inlet side, some of it has.
One very well known example of an engine utilizing the above techniques of combustion chamber "biasing" is the V-12 (double six) Jaguar previously used in the X-JS. I suspect you would have privileged knowledge of this engine extending well beyond mine. None the less, I believe the concept was instigated at the time by Michael May, allowing Jaguar to meet emissions and fuel economy standards previously not obtainable from the Heron type (combustion-cell piston) layout they started with. If I remember correctly the CR of that engine was/is over 11:1 and it runs comfortably on premium fuel.
I would be interested to know any more you may have to offer on the situation.
For those at home, the cylinder head was flat on the inlet side with the exhaust valve and spark plug housed in a "trench" making up the remainder of the "chamber." I recall the piston was a flat top. The heads were a single overhead cam type.
I would like to add that I've read quite a lot lately about the merits of a very shallow chamber with either a flat top or "mirrored dish" type piston, allowing the minimum surface area to volume ratio. This all sounds real good, but think of the following, which has apparently not been considered by the champions of the following ideals.
Taking the idea to the extreme, we find a chamber that is very wide and very thin with the spark plug in the center. This I'm told by varying experts would be "ideal!" What we have now is analogous to a flat sheet of paper! Try burning a flat sheet of paper and see how long it takes to get to the edges.
What I propose is instead confining the mixture into a volume of likely the same (but preferably less) area, but instead, we're going to shorten the distance from any given point relative to the spark plug by "shutting off" the inlet side (analogous to crumpling the paper). Indeed when the piston is down the bore, the surface area of it is greater than when at TDC. But when the piston is at TDC, the surface area of the "biased" piston dome would have to be less.
I proposed this technique to the originator of the post, as it's pretty easy to do with the pent-roof chamber design by simply angling the dome to closely coincide with chamber on the inlet side, while making the "slope" of the dome considerably steeper on the exhaust. I agree with Marquis about the squish clearance. You do not want to trap mixture somewhere on the inlet side, it must all be forced to the exhaust side or the design is a no-go.
Sean
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