Atkinson/Miller/West engine cycle(s)...
Atkinson/Miller/West engine cycle(s)...
(OP)
What's wrong with this..??
An engine that runs in the highly fuel efficient Atkinson cycle when low torque is required for simply cruising along at constant speed but then transitions into miller cycle when high torque is required, say for acceleration.
The "key" would be a variable intake valve closing delay. Have a smallish DFI engine with a static compression ratio of 12:1 but an expansion ratio of 15-16:1 during the power stroke.
Then use a variable speed positive displacement SuperCharger to boost engine output when acceleration is required. The throttle plate/valve could be eliminated.
As boost rises the intake valve delay would be increased to allow for the dynamic rise in CR due to SC boost.
What do yawl think..??
An engine that runs in the highly fuel efficient Atkinson cycle when low torque is required for simply cruising along at constant speed but then transitions into miller cycle when high torque is required, say for acceleration.
The "key" would be a variable intake valve closing delay. Have a smallish DFI engine with a static compression ratio of 12:1 but an expansion ratio of 15-16:1 during the power stroke.
Then use a variable speed positive displacement SuperCharger to boost engine output when acceleration is required. The throttle plate/valve could be eliminated.
As boost rises the intake valve delay would be increased to allow for the dynamic rise in CR due to SC boost.
What do yawl think..??





RE: Atkinson/Miller/West engine cycle(s)...
The BMW Valvetronic system also implements paragraphs #2 and #3 but in a fully variable manner, still subject to the compression ratio limitations.
Fiat also has a system called Multiair that implements #2 and #3, which looks like it will see production in the next couple of years.
All of these systems run at normal cam timing at full load so that the engine can develop normal torque output. No supercharger. The need to run normal cam timing at full load limits the maximum possible compression ratio, and the proposed use of supercharging would limit this even further.
The Toyota and Ford hybrids use the Atkinson cycle with a raised compression ratio and just accept the reduced power output - the hybrid system is used to make up for it.
Delaying intake valve closing (to reduce the amount of air going into the cylinder to reduce dynamic CR) isn't constructive if you are simultaneously trying to increase the amount of air by supercharging. You might as well not delay intake valve closing and not supercharge as much ... same outcome (and less power demand to run the supercharger).
I do not know of any current Atkinson-cycle implementations that also use direct-injection. It could certainly be done, but I suppose every additional system adds more cost and has diminishing returns, and it gets to a point where (at least currently) it isn't worthwhile. Emission regulations have dictated that direct-injection spark-ignition engines run at stoichiometric anyway, which defeats much of the promised efficiency improvement.
If the "supercharging" is replaced with "turbocharging" then some engine downsizing could be done. VW/Audi are doing this in combination with direct-injection, and it looks like BMW may be heading down that path also. These engines require the use of premium high-octane fuel.
RE: Atkinson/Miller/West engine cycle(s)...
You my be able to get a half point increase in actual static ratios by offsetting the cylinders relative to the crank centerline as the Toyota Prius engine does, but the rest has to come from valve timing.
First of all, there's lots of confusion and a lack of convention in literature as to the definitions of the Miller- and Atkinson cycles. Some define the distinction between the two as applying to force-induction and naturally-aspirated engines, respectively, with no distinction of late- or early- intake valve closure; others say Miller = late IVC resulting in push-back of some charge back in to the inlet manifold, while Atkinson = early IVC before BDC resulting in a partial expansion of the charge before the compression stroke takes place as usual. I tend to prefer and use the latter definition.
For low engine speeds and part-load, I would use early IVC and at higher speeds and high-load, use late IVC with charge boosting, thereby also taking advantage of gas dynamic effects to improve volumetric efficiency or at least offset the loss that results from charge push-back into the intake manifold that is associated with LIVC.
The static geometric compression ratio would remain some high value like around 10.5:1 (in a forced induction engine). More is feasible in a forced-aspirated GDI engine, but this limits the attainable knock-limited BMEP and requires more full-load AFR enrichening that negates the fuel economy benefits of GDI. This is precisely the subject I am currently undertaking active research: 2-stage turbocharged GDI engine that achieves a constant 26 bar BMEP from 1500-5500 RPM...
RE: Atkinson/Miller/West engine cycle(s)...
RE: Atkinson/Miller/West engine cycle(s)...
The Mazda used a positive displacement SC downstream of the throttle plate and ALWAYS provided boost when the throttle was open/cracked.
The same basic idea.
In order to turbocharge an engine its base efficiency, non-boost operation, must be sacrificed. The use of an variable speed SC allows efficient or BOOSTED operation throughout the operational engine range.
Using the Prius e/CVT technique if only atmospheric pressure is required the SC would simply idle along with very little engine drive or electric drive needed.
But yes, the "static" compression ratio by strict definition would need to be ~15-16:1 with DIVC bringing it down to 12:1.
Basically, except for DFI, we would have an engine equivalent to that in the Prius for simply cruising along and transition to one equivalent to the Madza Millenia S when power production became the mode.
RE: Atkinson/Miller/West engine cycle(s)...
The piston-and-valves-and-cams piece of the engine only sees "intake manifold pressure" in either case, it doesn't care how it was generated, and the turbocharger requires less parasitic loss from the crankshaft in order to operate it.
It's true that most traditional turbocharged production applications have been optimized for power rather than economy, but VW's TSI engines, Ford's Ecoboost engines, and GM's (hopefully) upcoming 1.4 litre turbo engine in the new Cruze are the other way 'round, and use turbocharging to allow the engine to be downsized.
RE: Atkinson/Miller/West engine cycle(s)...
Because turbochaging require one hell of a lot of WASTE energy entering the exhaust manifold and that does not happen at cruise speed engine torque levels.
Once BOOST "arrives" the cylinder charge will rise accordingly, so prior to boost arriving the CR must be inordinately low, lower than in a even a non-boosted engine.
Using an HSD e/CVT SuperCharging can provide boost, or not, all the way from idle RPM up to the rev limiter.
TurboCharging only works when there is enough energy left in the exhaust stack to move the turbine. Use the Atkinson cycle, or Miller, and more of the energy of combustion is used up in pushing the piston downward with NOTHING left over for spinning a turbo.
Ford is done nothing other than hiding the "wizard" behind the curtain. The DFI CR advantage could be put to greater use.
RE: Atkinson/Miller/West engine cycle(s)...
Current practise of typical EVO timings are also in the order of 40-70° BBDC. You could open the exhaust valve very near BDC, but whatever you might gain in indicated efficiency will be lost in increased pumping work during the exhaust stroke and increased residual gas mass fraction for the next cycle hindering combustion.
RE: Atkinson/Miller/West engine cycle(s)...
Starting with a mechanical CR of ~15-16:1 but in Atkinson mode using delayed valve closing to make the "effective" CR about 12:1 (DFI). Then as boost rises the valve closing delay would be extended as needed to prevent knock/ping.
RE: Atkinson/Miller/West engine cycle(s)...
RE: Atkinson/Miller/West engine cycle(s)...
- Steve
RE: Atkinson/Miller/West engine cycle(s)...
RE: Atkinson/Miller/West engine cycle(s)...
RE: Atkinson/Miller/West engine cycle(s)...
But to get back to Wwest's proposition: - you can't even run anywhere near full atmospheric manifold pressure with an Atkinson engine let alone above atmospheric as with supercharging. Really I think this is what Ivymike pointed out, and there is no way around it.
I think Miller cycle engines with their superchargers have confused the issue - I don't think this is true supercharging in the normal sense. The pressure in the combustion chamber after compression is the critical factor no matter what type of cycle is being used or how you arrive this point.
So no - I don't think you can change from Atkinson Cycle to Miller Cycle.
RE: Atkinson/Miller/West engine cycle(s)...
1) Hand waving (very popular).
2) Hardware (expensive).
3) Simulation.
- Steve
RE: Atkinson/Miller/West engine cycle(s)...
But, why not...??
4 cylinder Atkinson cycle engines exist, work, and are in current production and use.
Miller cycle engines exist, work, and are in current use.
DFI engines exist, work, and are in current production and use.
Engines with VVT and VVT-i exist, work, and are in current production and use.
Apparently switching a 4 cylinder engine between the two, Atkinson vs Miller, might involve something as simple as an blower type SC with a clutch.
Aftermarket addition of an SC or Turbo to an otherwise off-the-shelf, standard, production engine is a fairly common event.
Obviously one could optimize, increase the level of boost possible, with the aftermarket addition of an SC or blower via reduction of the engine's CR. But that would result, also obviously, in a lower FE "off-boost".
All I'm suggesting is an explansion of teh use of VVT to accomplish a dynamic change in the CR as/when the SC begins producing boost.
RE: Atkinson/Miller/West engine cycle(s)...
The part about supercharging a high compression engine causes detonation.
The part that reducing compression by late intake valve closing reduces VE and power.
The part that by correcting that with supercharging only gets back the VE that you deliberately lost by late valve closing, but still suffers the parasitic losses of of driving a supercharger.
The part that no matter how you achieve the cylinder pressure, you can only tolerate a certain amount before it detonates.
Regards
Pat
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RE: Atkinson/Miller/West engine cycle(s)...
I think a lot of magazine writers etc. have confused the issue by giving the false impression that the Miller Cycle allows a high mechanical CR (and expansion ratio) to be used by having LIVC to to reduce charge pressure and then regain all that has been lost by supercharging. This is really not what the Miller Cycle is.
I think the most surprising thing is that Ralph Miller realized that the combination of blower and piston took less energy than piston alone to get the final pressure.
RE: Atkinson/Miller/West engine cycle(s)...
The second part of the equation has to do with the fact that the air compressed by the SC can be COOLED post-compression.
So the charge reaching the cylinder will not be as hot as it would be were it compressed solely by the piston travel.
RE: Atkinson/Miller/West engine cycle(s)...
I think the idea with the Miller cycle is that during light to moderate load operation (95+% of most people's driving) the supercharger does not operate (and does not cause any meaningful power losses) so the engine can operate with reduced throttling losses. At full load, the supercharger makes up for the volumetric-efficiency disadvantage (it is NOT a high-pressure-ratio supercharger if I remember right - the power output of that engine was nothing special) to allow the engine to have a normal power output. But, by stuffing extra air into the engine it's still subject to limitations imposed by detonation, so the compression ratio can't be raised above normal. At full load, I would expect the power demand to run the supercharger to make the whole package slightly LESS efficient than a conventional unsupercharged engine. But, most people's driving spends so little time at full load that it really doesn't matter too much to the actual fuel consumption in normal driving.
The Civic engine (and the BMW Valvetronic) reaches a normal power output by varying the intake valve timing and lift but again the compression ratio has to remain "normal" because of detonation limitations at full load.
The Prius engine and the Atkinson version of the Ford 2.5 engine (used in Escape and Fusion hybrids) have delayed intake closing with a raised compression ratio and no supercharging and they deal with the reduced power output of the engine by using the hybrid system to make up for it.
Higher pressure supercharging whether "super" or "turbo" can be used by using very good intercooling without sacrificing the compression ratio too much.
Regarding not having enough exhaust energy left to run a turbo ... fat chance. In a normal application if the engine is to have anywhere near an acceptable power output, you won't be able to have enough difference between the effective compression and expansion ratios to use up ALL of the extra energy during the power stroke. Diesel engines already use up more of the energy to drive the piston than gasoline engines do, and automotive applications are universally turbocharged and intercooled (not mechanically supercharged) nowadays. It requires different calibration of the turbine side than a gasoline-application turbo does, that's all.
RE: Atkinson/Miller/West engine cycle(s)...
Again, the idea is to use the highly fuel efficient Atkinson cycle mode at those times an "acceptable" power output is NOT required. Then when an "acceptable" level of power IS required then made use of the variable speed aspect of the SC and further closing delay of the intake valve to prevent the CR from rising to a level that would result in detonation.
Which leaves the question of what CR can be used.
We all now know, or certainly should know, the DFI engines can accomodate a CR in the range of 12.5:1 without incurring detonation. Add in the fact that the "pre-compressed" airflow from the SC can also be "pre-cooled" and it should be clear that the CR could possibly approach 13-14:1.
RE: Atkinson/Miller/West engine cycle(s)...
I propose that the name "BigVlad-West Cycle" be used for this type of engine.
RE: Atkinson/Miller/West engine cycle(s)...
And it appears to me that unless the engine is a 4 cylinder or multiples thereof the SC must be of the positive displacement type. Otherwise you would probably need a backflow provention valve (reed/shuttle valve ??) much like those in use in 2 cycle engines.
RE: Atkinson/Miller/West engine cycle(s)...
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Assuming the compressor (mechanical or turbo) was able to reach a high pressure ratios at high efficiencies, one might also consider to use a 2nd intercooler after the first intercooler to further reduce temperature at TDC and subsequently be able to increase pressure even more.
This 2nd intercooler was "powered" by a cold reservoir powered by an powerful air-conditioner (which is also used to cool the passenger compartment), which would mainly run during braking and low loads and turned off at high loads. (The 2nd intercooler was only utilized at full load for a short time and due to the sudden density increase would also release a sudden power increase.)
At least storing "cold heat energy" is significantly cheaper than storing the same amount of energy in a battery, but it would obviously be also less efficient.
The question is as always: Would the efficiency and/or power density gains worth the increased costs of the system? (Maybe not)
Btw: The miller cycled Mazda had no intercooler:
h
RE: Atkinson/Miller/West engine cycle(s)...
Morg
RE: Atkinson/Miller/West engine cycle(s)...
Good read, but keep in mind that this was for a cogeneration engine and did not need to operate throughout the RPM and power range of the typical automotive environment.
Text also makes it quite plain that tubocharging is NOT compatible with the Miller cycle for automotive use.
Anybody thought of carrying a canister of medical LOX on board for POWER surges...??
RE: Atkinson/Miller/West engine cycle(s)...
besides that this was not my point and I did not read anything in the article stating that miller cycle and turbochargers are not compatible in automotive applications:
Actually, BorgWarner is working on a turbocharged diesel engine with variable valve timing to benefit from the Atkinson cycle:
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And as BrianPeterson pointed out: Diesel engines already use up more of the energy to drive the piston than gasoline engines do, and automotive applications are universally turbocharged and intercooled (not mechanically supercharged) nowadays.
Besides, why even care whether a miller cycle has a turbocharger or a mechanical compressor or an electrical compressor?
RE: Atkinson/Miller/West engine cycle(s)...
Because in order to spin the turbine portion of the turbocharger a significant level of power must go into the exhaust manifold. Since the Atkinson or Miller cycle engines make greater use of the combustion of the air/fuel mixture there is little likelihood of having enough left over to spin a turbine.
RE: Atkinson/Miller/West engine cycle(s)...
FULLY expanding the power stroke in the cylinder would require the effective power stroke to be more than double the compression stroke. This would reduce volumetric efficiency by half, i.e. the engine would need to be twice the size of a regular one, with appropriately higher friction losses, etc. And, it would result in *over* expanding the gases when the engine is running at part load, and there is just as much loss by overexpanding than by underexpanding. In an automotive application it's generally better to optimize the part-load operation for efficiency even if it means sacrificing a few points of efficiency at full load.
So, a more realistic solution might be (for example) to make the effective compression stroke three-quarters of the power stroke. This would make the amount of expansion "about right" at part load (the turbocharger would be ineffective, but it doesn't matter, it's PART LOAD), but would leave some energy left in the exhaust to operate the turbocharger at full load.
It will probably be found that the reduced frictional losses and reduced thermal losses from downsizing and turbocharging will be more favorable than using a huge non-turbocharged engine, IF it is optimized for fuel consumption rather than just power output.
RE: Atkinson/Miller/West engine cycle(s)...
If you're going to put "power" into spinning a turboharger why not instead put an equal level of power into spinning a positive displacement SC...??
No turbo lag whatsoever, the expansion ratio can be more highly optimized ignoring the need to spin a turbine, and the throttle plate could be eliminated.
RE: Atkinson/Miller/West engine cycle(s)...
Ask yourself why EVERY 4-stroke production automotive / truck diesel engine uses a turbocharger, not a mechanical supercharger. Every. Single. One.
RE: Atkinson/Miller/West engine cycle(s)...
Other than parasitic losses how is it possible for a turbocharger to be more efficient than a SuperCharger...??
Or let's take a simpler approach.
One of the main problems with a turbocharger is the time it takes to spool up once I ask the engine to produce an abundance of power. If I use more energy to push the piston down during the expansion cycle doesn't that increase the turbo lag..??
And we all know an SC has NO lag.
RE: Atkinson/Miller/West engine cycle(s)...
I ask again, if your line of thinking were correct, then why does EVERY modern 4-stroke diesel engine use a turbocharger, and not a supercharger?
Modern low-inertia turbochargers have very little lag.
If you use more energy to push the piston down, it just means you will have to reconfigure the exhaust turbine to extract more of the energy. Turbochargers on gasoline engines are deliberately rather open on the exhaust side, to make use of less of the energy in the exhaust, because otherwise they would overboost. Turbochargers on diesel engines are calibrated differently to extract more of the energy in the exhaust, because there is less of it to begin with.
RE: Atkinson/Miller/West engine cycle(s)...
Studebaker did this in back in the mid-fifties in their Golden Hawk series using a v-belt type CVT.
Nowadays one would use the e/CVT concept from the Toyota HSD system. Basically a differential drive with one input being the engine and the other a permanent magnet rotor AC synchronous motor powered by a variable frequency AC inverter much like that used to drive the Prius' A/C compressor.
Since it is quite common for an AC motor of this type to be able to turn up to 20,000 RPM vs ~6000 RPM for the engine a planetary gearset instead of a true differential might be more appropriate. A 2HP electric motor controlling the speed of an SC that also has 10HP from the engine..
..OR NOT.
The electric motor could negate any or all HP coming from the engine.
If one used a positive displacement SC then the throttle plate could be eliminated.
RE: Atkinson/Miller/West engine cycle(s)...
The VVT-i description for the 2010 RX450H and Prius is now available at techinfo.toyota.com.
The newest hybrid engine designs actually use VVT-i to move the intake valve opening delay into and out of the Atkinson cycle mode. With low engine loads/loading, low charge going into the cylinders, the VVT-i is set to NOT use delayed intake valve closing and thereby hold the compression and power strokes to 13:1, 12.5:1 for the RX.
Smart.
RE: Atkinson/Miller/West engine cycle(s)...
So where exactly does the power to drive the electric motor come from?
- Steve
RE: Atkinson/Miller/West engine cycle(s)...
Why bother. The credibility is now clearly established, or not.
Regards
Pat
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RE: Atkinson/Miller/West engine cycle(s)...
The electric motor can negate any and all "TORQUE" produced by the engine.
RE: Atkinson/Miller/West engine cycle(s)...
Vary the rate at which the electric motor spins its input shaft in the "opposite" direction and the output shaft RPM will vary accordingly, with the engine RPM held constant.
Give the electric an "advantage" via a planetary gearset, say 4:1, and a 2HP (20,000 RPM max) electric motor can hold "sway" over an 8HP engine drive input.
Anyway, that's how the Toyota HSD systems' e/CVT operates.
RE: Atkinson/Miller/West engine cycle(s)...
Internal combustion engines are at a similar point to where electric motors were 25 years ago.
High efficiency units are now available but we quickly find that the higher efficiency motors have preferred areas of operation. Operation off peak results in no efficiency gain and even lower efficiency.
The exceptional fuel performance demonstrated by the Prius is not because it is a hybrid but rather that it operates its engine in its area of peak efficiency. The concept of differentially countering the engine with an electric motor/generator was patented by TRW in the early 70s. Back then we really did not have good low cost inverter technology. We still don't but we are better off than back then. But this method does allow you to operate with a smaller internal combustion engine and also a smaller electric motor/inverter. Since the engine is smaller it operates higher up on its efficiency curve. The smaller electric motor and associated electronics allow further cost saving.
RE: Atkinson/Miller/West engine cycle(s)...
~2500 RPM limitation.
RE: Atkinson/Miller/West engine cycle(s)...
In aeronautical applications it seems like turbo compounding would be the better approach to achieve additional expansion needed at high throttle setting. Put a motor/generator on the turbo shaft. This would be used as a motor to spin up the turbo fast and to aid in starting the engine. Then when the engine is run at full or near full power, extra turbine power would be absorbed by the generator as in turbo compounding.
RE: Atkinson/Miller/West engine cycle(s)...
Wasting energy at the very time it is most needed.
With a variable speed/volume SC no "wastegate" equivalent would be needed.
RE: Atkinson/Miller/West engine cycle(s)...
RE: Atkinson/Miller/West engine cycle(s)...
Any heat energy recovered from the exhaust to drive the turbocharger is bonus in an energy balance of the powerplant that, aside from the negative piston work from the added backpressure, you can never recover from a mechanical supercharger. In a well-matched operating point in a turbocharged engine, the boost pressure can be higher than the exhaust manifold pressure (positive piston PdV work), and the turbocharger setup can result in a net positive efficiency balance. A supercharger is ALWAYS negative for efficiency because it directly takes useful work from the crankshaft to drive, regardless whether there are any variable drives or not.
Superchargers are used because there isn't the lag associated with turbochargers. They also have advantages for underhood thermal-loading and possibly packaging as well. Superchargers have places in applications where best-possible transient performance is demanded and efficiency is secondary.