Throttleless propane, succesful test and idle discussion
Throttleless propane, succesful test and idle discussion
(OP)
Some of you may remember I have been looking into running my new motor on propane, and not using any sort of air throttle. Goal was to regulate power through fuel delivery, taking advantage of propane's excellent lean burn limits and near perfect atomization.
Before any testing I knew the biggest hurdle would be idle. The fear was that power could not be reduced enough through a super lean mixture to keep the engine at a reasonable idle speed. Beyond this, it may have turned out the mixture couldn't even be reduced enough for mild load situations (steady cruise, light acceleration).
The test I conducted this weekend put these fears to rest.
The test motor was a 3L, V6, 8.5:1 compression, motor on my wife's car (93 Dodge Shadow). Note the intended usage motor will be a 3L, I6, 10:1 compression. The motor was warmed up on gasoline. Then shut down. Fuel injectors were disconnected, and the throttle stuck wide open. A 500000BTU/hr propane torch was then inserted into the open throttle. While the engine was being cranked over, I opened the valve on the torch. After about 1 turn, the motor started without hesitation. It did not run away. Rather it stayed at about 800RPM. I then experimeted for a minute, increasing and decreasing the propane flow, and noting I had perfect control of the idle speed. More propane, motor sped up. Less, it slowed right back down. It never stalled, popped, or backfired. Finally shut off the valve and the engine instantly shut down. I do note however, that it ran a little rough, with a slight misfire.
My conclusions from this test:
Load control through fuel delivery won't be a problem. The low end of the lean limit range without misfire goes very low indeed. I have no doubts I can have mild accleration and steady cruise on all 6 cylinders without misfire or rough running. Perhaps, with the improvement from the test 8.5:1 to the 10:1 of the motor the full system is intended for, may be able to lean idle dead steady without any misfire. Also consider I had a fixed ignition advance of 15BTDC on the test motor, and will have complete control over timing on the actual motor. This could help as well. I'm not counting on that however.
I have two other options available to me to keep the motor down at a idle speed when desired.
1. Fuel cut rev limiter. I'd set my fuel map so that at 0% throttle input and 1500RPM, the fuel injected would be at the very lean limit of steady, no misfire operation. However, this will probably be slightly more power than what is needed to keep the engine purring along at 1000RPM with no external load and full atmospheric pressure in the intake. I'd use a second rev limiter (available on my fuel computer) to kick in a fuel cut only, to keep the revs down. The limiter is very adjustable, having a 0-100% level of effect in two seperate stages, as the engine revs above the targeted limit. It wouldn't be a terribly hard rev limit like you are used to thinking about either, as most likely less than 25% of injection events will need to be dropped out to keep the revs steady, rather than the 75-90% of injection events on a regular engine running full stoich or richer. Before you mention, the propane is individually port injected, which is why I can do this.
2. Run the engine on two cylinders at 0% throttle. Simply run the engine on cylinders 2&5 only (firing order 360 apart). My fuel computer can easily be made to do this as well. To maintain idle speed, I'd guess those two cylinders would probably need to run at 75-100% of stoich. Anyone care to comment if this would cause harmful vibrations, harmonics, or some other concern?
Before any testing I knew the biggest hurdle would be idle. The fear was that power could not be reduced enough through a super lean mixture to keep the engine at a reasonable idle speed. Beyond this, it may have turned out the mixture couldn't even be reduced enough for mild load situations (steady cruise, light acceleration).
The test I conducted this weekend put these fears to rest.
The test motor was a 3L, V6, 8.5:1 compression, motor on my wife's car (93 Dodge Shadow). Note the intended usage motor will be a 3L, I6, 10:1 compression. The motor was warmed up on gasoline. Then shut down. Fuel injectors were disconnected, and the throttle stuck wide open. A 500000BTU/hr propane torch was then inserted into the open throttle. While the engine was being cranked over, I opened the valve on the torch. After about 1 turn, the motor started without hesitation. It did not run away. Rather it stayed at about 800RPM. I then experimeted for a minute, increasing and decreasing the propane flow, and noting I had perfect control of the idle speed. More propane, motor sped up. Less, it slowed right back down. It never stalled, popped, or backfired. Finally shut off the valve and the engine instantly shut down. I do note however, that it ran a little rough, with a slight misfire.
My conclusions from this test:
Load control through fuel delivery won't be a problem. The low end of the lean limit range without misfire goes very low indeed. I have no doubts I can have mild accleration and steady cruise on all 6 cylinders without misfire or rough running. Perhaps, with the improvement from the test 8.5:1 to the 10:1 of the motor the full system is intended for, may be able to lean idle dead steady without any misfire. Also consider I had a fixed ignition advance of 15BTDC on the test motor, and will have complete control over timing on the actual motor. This could help as well. I'm not counting on that however.
I have two other options available to me to keep the motor down at a idle speed when desired.
1. Fuel cut rev limiter. I'd set my fuel map so that at 0% throttle input and 1500RPM, the fuel injected would be at the very lean limit of steady, no misfire operation. However, this will probably be slightly more power than what is needed to keep the engine purring along at 1000RPM with no external load and full atmospheric pressure in the intake. I'd use a second rev limiter (available on my fuel computer) to kick in a fuel cut only, to keep the revs down. The limiter is very adjustable, having a 0-100% level of effect in two seperate stages, as the engine revs above the targeted limit. It wouldn't be a terribly hard rev limit like you are used to thinking about either, as most likely less than 25% of injection events will need to be dropped out to keep the revs steady, rather than the 75-90% of injection events on a regular engine running full stoich or richer. Before you mention, the propane is individually port injected, which is why I can do this.
2. Run the engine on two cylinders at 0% throttle. Simply run the engine on cylinders 2&5 only (firing order 360 apart). My fuel computer can easily be made to do this as well. To maintain idle speed, I'd guess those two cylinders would probably need to run at 75-100% of stoich. Anyone care to comment if this would cause harmful vibrations, harmonics, or some other concern?





RE: Throttleless propane, succesful test and idle discussion
I would recommend one additional test. Try it with a motoring load to simulate drivetrain loads. Propane has a lean and rich combustion limits of 2.1-9.6% but can operate at regions outside of that range, but not too far. Do you have any idea of what the actual fuel use was at that load?
The misfire you indicated was the situation I discusses earlier, some cylinders will have fuel, others will not, or an insufficient amount.
When you can obtain a rock steady idle under standard loads at WOT, you will have my nod of confirmation. The rest of your discussion seems valid, assuming the above test works.
One thing to consider: At WOT, and the fuel mixture being the only speed limiting factor, ANY stray fuel anomaly will result in a run-away. Just ask any diesel mechanic who washed the air filter in solvent and relied on engine air to dry it out!
Franz
RE: Throttleless propane, succesful test and idle discussion
I couldn't tell you the exact fuel use (didn't have a flowmeter), but it was right in line with my previous calculations. I had already determined that a 500000BTU/HR torch was perfect, allowing me to match the amount of fuel used normally during idle, with a little extra to get it higher. So I don't think I was wasting a great deal of fuel.
RE: Throttleless propane, succesful test and idle discussion
500,000 BTU/hr works out to about 200 cu ft of raw propane per hour (2500 btu/cu/ft). Assuming your 3.0 engine was at 800 rpm, that would account for about 2500 cu ft air per hour, or about 12.5:1 air to fuel ratio by volume. This is about half of propanes ideal fuel mixture of 24:1 volume (15.5:1 AF by weight) Something doesnt make sense. Was the torch open full blast?
Please dont get me wrong, I really like your approach and applaud your efforts. This has been bantered about in lab engines for many years, myself included. I was unable to obtain transparent operation and have usable low speed engine power, although I could nurse an engine to low speeds at ridiculously high AF rations.
Franz
RE: Throttleless propane, succesful test and idle discussion
The torch wasn't open full blast. It had a fine adjustment knob, and I'd guess it was only open at 10-20% flow at the slow 1000RPM idle.
I'm sure this wouldn't work for "transparent operation" nessasary for widespread passenger vehicle use. That's a higher standard than I require. If I can nurse the engine to low speed at the far lean limit, which I am reasonably sure I can do, that's enough. The idle can require a little bit of fuel cut (removal of 10-25% of injection events), and warble a little. No harm in that. Most likely it will just sound more "cammed" in the end.
This has major potential as a race car, "off-road use only" application.
By having load control be entirely electronic, I am free to further experiment with other things later. In the immediate term I won't need a throttle, a blow off valve, or any sort of idle control valve. I could try incorporating a smooth traction control, where the mixture is leaned out rather than having a rougher fuel or ignition cut as traditionally used. I probably won't need a wastegate on the second stage turbo of the compound setup, rather I could just stop injecting more fuel past a certain boost level, and it will lean out a bit (and cool the exhaust), and the boost will stop rising.
RE: Throttleless propane, succesful test and idle discussion
I keep thinking there's got to be a better way to run a spark ignition engine.
With respect to your misfire you might find a benefit from using individual injectors although there may be issues with the range of fuel flow available to cover idle to full power.
It would be interesting to see what the combustion variability is under these super lean conditions - that's one area where CI engines score over SI.
Keep up the good work.
Paul
RE: Throttleless propane, succesful test and idle discussion
Paul
RE: Throttleless propane, succesful test and idle discussion
550cc/min injectors (rathed for gasoline at 43.5psi). I figured they will flow far more with 120psi and liquid propane, enough to cover my fuel demands.
They are low impedance, and a side flow design. I'm hoping the side flow design, which has the pressurized fluid moving around the main body of the injector, will prevent injector failure usually associated with high pressure. I'll be sure and update here after I can test them.
RE: Throttleless propane, succesful test and idle discussion
You will quickly encounter differences in fuel handling between the torch in vapor phase, and liquid phase. Remember that when shutting off the engine, the injectors will absorb heat and pass it through to the injectors. I have seen fuel rail pressures exceeding 400 psig, due to heat saturation. I have also seen the aftermath of blown up injectors and fuel lines.
Again, be cautious. Use due diligence and VERY thorough and sound engineering when attempting liquid phase injection, not to even get into the throttleless design.
Franz
RE: Throttleless propane, succesful test and idle discussion
Help me understand this high pressure in the lines problem you mention. I thought about it and I don't quite get how the problem occurs.
So you have some heat get in, and it raises the temperature of the liquid propane. Some of it boils, but as it does it does two things. Expand, which would increase pressure, which just happens prevents further boiling. It also cools, would would also, stop the boiling. To get to 400psi, the propane itself has to be at over 200F, or it would be a liquid.
I should note, it will be a very cool injection in operation. It will be mounted in a intake manifold on a I-6 engine, so exhaust heat is on the other side. The exhaust header and exhaust will all be fully heat insulated. The car will have a heavily vented hood. The intake manifold will be fully insulated from the engine compartment (encased in low heat conduction plastic shell), and be insulated from the head with a phenolic spacer. And, the propane will be cooling off most right at the base of the intake manifold, so if anything under normal operation the propane will be getting cooled, not heated.
As I see it, if you keep heat away from the propane, which I will, there isn't a problem.
So the problem would only lie on shutdown, and heat soak, though minor it would be in my case. Since I won't need nor use a check valve of any sort in between the tank and the rail, any expansion or boiling at the rail will just displace back into the tank, where it will cool and recondense.....I think. Then, next time the engine is started, the vapor in the line will purge through the injectors almost instantly. I imagine this approach is no good for widespread use, as a tiny amount of propane is likely to go out the tailpipe unburned as a result, but fine by me.
RE: Throttleless propane, succesful test and idle discussion
From an injector point of view I would echo Franz's comments - I don't think they will survive too long - apart from the pressure, the lubricity of liquid propane is significantly worse than gasoline.
I think Tickford did a liquid propane injection system for the European Ford Focus and Transit vehicles but had problems with the injection control particularly when regulating small amounts of fluid such as at idle. They also had issues with ice forming in the intake. You might be able to find some data from these programmes to help you.
As an alternative you could look at using vapour injection which would probably make idle and part load a lot simpler to regulate. There are quite a number of vapour phase injectors on the market, you would need a vaporiser in the system though which would complicate things.
RE: Throttleless propane, succesful test and idle discussion
Propane is a pressurized liquefied vapor. The vapor pressure index describes what pressure is required to keep propane a liquid. This information is available in most engineering handbooks, especially ones that deal with LPG, and in NFPA 58.
Lets assume the tank temperature is 70 deg F. Your pressure will be around 120 psig. At 100 deg F, it would be around 190 psig. At 50 deg F, it would be around 85 psig.
LPG will indeed boil, but if the line is liquid saturated, and closed at both ends, it will expand until the line or injectors rupture. It will not auto-refrigerate unless the vapor can be released to a larger space. Also, if the fuel returns to the tank, it will expand, but it will not condense and refrigerate, just transfer the heat from a small area to a larger area (First and Second laws). I have seen liquid injection fuel tanks hot enough to blister your hands! ! !
This is the primary problem with the production LPG liquid injection systems. Tank pressures approaching the pressure relief range is not uncommon. Hot engine restart is difficult.
Restarting the engine, purging the vapor in the line (you stated you have a return loop, which fuel is pulled, vapor or liquid? You will not be able to operate the liquid injectors with the fuel map in vapor phase (270 times reduction in volume) and vice versa.
Pbloore comments are also valid.
Now, if you were just using vapor injection. . . .your biggest problem is sourcing injectors. Bosch and Delco both make CNG injectors, I would start with Bosch.
Regards;
Franz
ps: I would be happy to mail you a vapor pressure graph, send me your request to:
franzh "the @ symbol" texas.net
RE: Throttleless propane, succesful test and idle discussion
I considered solenoids of some sort, but haven't found any that can open and close as fast as injectors. If anyone knows of some solenoids that might fit the bill, I'm eager to see them.
RE: Throttleless propane, succesful test and idle discussion
The system I'm using is manufactured by BRC who have a tie up with Impco, so I would have thought the bits should be available in the USA.
I think you'll find a vapour system a lot easier.
Paul
RE: Throttleless propane, succesful test and idle discussion
One thought, vapor propane flow through any metering devise goes up significantly with pressure, because in addition to the sqrt(P1/P2) difference, you also have the propane becoming denser with pressure, which liquid fuels don't do. So if the vapor injectors you are using can handle it, I could just get by with more pressure.
RE: Throttleless propane, succesful test and idle discussion
Find out who they will sell to and why, like spare parts for instance. You might need a specific application for the spares.
You might need an LPG fitters licence of some sort, or a friendly fitter as co-conspirator, then tell them what they want to, or need to hear. They may have legal restraints re safety, licences and liability, and a major fear of lawsuits, or criminal action. If you can eliminate these fears by taking the risk yourself, they may get a somewhat more willing.
I should not advise you to lie, but I am sure, that if there is a will there is a way, and you will figure it out
Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
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RE: Throttleless propane, succesful test and idle discussion
The injectors themselves are rated at up to 30 kW/cylinder natuarlly aspirated or 38 kW/cylinder turbo. I would have thought therefore that you could certainly use an injection pressure of between 1.5 and 1.9 bar without any problem. This should give you a capability of about 300 HP on a V6 with a single injector per cylinder and two vapourisers.
Regards
Paul
RE: Throttleless propane, succesful test and idle discussion
RE: Throttleless propane, succesful test and idle discussion
You need to keep track of the vapor pressures and the Joules Thompson effect. How do you plan on supplying the varying pressures depending on boost levels?
In the US, many suppliers will not sell these specialized products to the general public due to liabilties. I have a close working relationship with many of them and I have to jump through hoops to obtain certain components. I often find it easier to go overseas and get what I need, but frequently lose some technical advantage then.
Many overseas manufacturers deal with smaller engines, thus smaller orifice sizing and flows.
Franz
RE: Throttleless propane, succesful test and idle discussion
160lb/hr injectors, by my calculations (software SF Pressure Drop), will each flow aprox 25-28lb/hr of vapor propane at 80psi (95psi absolute) rail pressure. That's enough for starting, idling, cruise, and light acceleration (less than 10psi boost).
I could have a sealed stainless pressure tank, pressurized by the same regulated pressure of the supply (80psi). This would drain out of the bottom of the tank, and lead to a solenoid, which is then connected right at the inlet of the fuel rail. At the flip of a switch, a solenoid in the propane line will close, and the solenoid for the methanol will open. The propane vapor will quickly purge out of the rail, and it becomes a methanol fuel system. I'd also have the fuel computer cut the duty cycle as well, as I'll need only about 40% as much duty to get the same lambda AFR with liquid 80psi methanol, compared to vapor 80psi propane.
I'd think, methanol would make more power than propane anyway, and be a better fuel for full power boost. I'd probably run with 5-10% water content as well.
Propane is a great fuel for all the other conditions (starting, idling, cruising, light acceleration), and I should still be able to run throttleless that way. The other bonus is I won't need to purge the fuel system of methanol after use, the propane will do that the instant I switch the solenoids back again.
In terms of controlling power when in methanol mode without a throttle, and I don't think I'd want to let it run lean, I was thinking of having the same ~25% fuel cut I'd use to maintain propane idle speed, also come into effect if I back out of the throttle at full boost. Throttle position drops below 90%, instigate a 25% fuel cut (meaning 1 out of 4 injection pulses), throttle position drops below 60%, cut fuel altogether.
RE: Throttleless propane, succesful test and idle discussion
I'll be able to flow enough propane to support up to 39lb/min air at stoich. That's quite a bit. 390HP or so. Should cover all highway use passing situations. If I run out of flow as boost and RPMs rise, it will just go leaner, dropping the boost, and keeping the HP output near static.
I have read from a USC engine project on throttle engines, that methanol is also a very good lean burn fuel, perhaps as good as propane. So, if I was to switch fuel modes, I won't be suffering trying to get load control low with fuel mix. Maybe the "just above misfire" threshold will be step higher, which I can contend with. So I can run it lean, just not once I get into boost. Once I'm in that danger region, I'll use the fuel cut (25% or 100%) to back off power.
RE: Throttleless propane, succesful test and idle discussion
With respect to running methanol through the same injectors - I think you'll have problems, partly down to the characteristics of methanol itself, but also from trying to mix liquid and gas fuels. I would suggest that using two separate fuel systems would be a simpler system overall. My car is Bi-Fuel so has the original OEM gasoline injection system plus a propane injection system as well. Changeover occurs when the propane temperature reaches a threshold and is done by switching over the fuel in each cylinder one at a time. I think I set it up to changeover one cyclinder, wait three revolutions, then do the next and so on. This works very effectively and in use is almost transparent. There is an automatic switch back to gasoline if the regulation pressure drops below a threshold - the only time this is noticeable is when it occurs during fairly hard accelerations.