Atkinson/Miller/West engine cycle(s)...

[wwest]

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..??

 

[wwest]

"...fat chance...if the engine....an acceptable power output..."

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.

 

[BigVlad]

I have to change my mind somewhat on this matter. I now think wwest's idea is interesting. However rather than his idea of changing between Atkinson and Miller cycles I would suggest applying both Atkinson and Miller cycles simultaneously. As both involve late closing of the intake valve this should be possible. As the Miller is essentially a compression stroke technique and Atkinson is concerned with the expansion stroke, the two cycles would not interfere with each other. The efficiency gains from both Atkinson and Miller cycles should be cumulative to make quite an improvement in overall efficiency. My idea would be that the geometric CR would be maybe 16 or 18:1 and that the pressure in the combustion chamber would be limited by LIVC to accepted levels. Basically the controlling cycle would be Atkinson with the air/fuel mixture supplied Miller-Cycle fashion - by blower etc. The total gain in efficiency of possibly 20+% would make up to a large degree the torque/power that was lost by having to limit the amount of mixture Atkinson-style. If the A and M cycles were combined with Helical cam-style control of the idle and low engine load (which is yet another LIVC technique) it would make an extremely fuel efficient engine.
I propose that the name "BigVlad-West Cycle" be used for this type of engine.

 

[wwest]

Other than the fact that Mr. Miller already incoporated, incorporates, the Atkinson cycle as a "building block" for the Miller cycle engine your description sounds pretty exacting of my theory.

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.

 

[globi5]

Here's a paper with "miller cycled" turbocharged engine:

http://www.mhi.co.jp/technology/review/pdf/e383/e383146.pdf

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:

http://www.motivemag.com/Content/uploads/1/miller1_center.jpg

 

[morg]

I need an electrically controlled valve or method to move 250 cubic inches of compressed air at 4000 psi in 10 milliseconds or less. Off to On to Off again. In 0.010 Seconds. Is it possible? Are there air valves of that speed? Alternate methods?

Morg

 

[wwest]

globi5:

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...??

 

[globi5]

wwest,

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:

http://www.atzonline.com/index.php;...85207104a2011a787f90916594486/alloc=3/id=9598

http://www.sae.org/automag/features/futurelook/08-2005/1-113-8-74.pdf

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?

 

[wwest]

"...why even care.."

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.

 

[BrianPetersen]

^^ Don't count on it.

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.

 

[wwest]

Now that we know, have verified, that the power to spin a turbo need not otherwise be wasted...

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.

 

[BrianPetersen]

Turbo is more efficient. Only a portion of the energy delivered to the turbine comes from the extra back pressure during the exhaust stroke. Most of it comes from the heat and pressure that you would otherwise be throwing down the exhaust pipe.

Ask yourself why EVERY 4-stroke production automotive / truck diesel engine uses a turbocharger, not a mechanical supercharger. Every. Single. One.

 

[wwest]

Isn't there some "law" that says if you want to compress a given volume of air to a specific pressure level it ALWAYS take an equal amount of energy...??

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.

 

[BrianPetersen]

A turbocharger is making use of energy that is (for the most part) not otherwise useable. A supercharger is taking away shaft horsepower that your engine already worked hard to generate.

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.

 

[wwest]

I strongly suspect that Superchargers are not more commonly used because up to now there was no obvious way to have the engine be the primary SC driving force but still have variability of SC speed/boost independent of engine speed.

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.

 

[wwest]

New...NEWS...!!

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.

 

[SomptingGuy]

"The electric motor could negate any or all HP coming from the engine."

So where exactly does the power to drive the electric motor come from?

- Steve

 

[patprimmer]

Steve

Why bother. The credibility is now clearly established, or not.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[wwest]

Sorry..

The electric motor can negate any and all "TORQUE" produced by the engine.

 

[wwest]

Look at, consider, the differential driving the SC as a "summing" device. If one input is spinning at 3000 RPM and the other input is also spinning at 3000 RPM in the "opposite" direction then the output shaft will be stationary. Simply "lock" the electric motor in place and the output drive will become a function solely of the engine, ICE.

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.

 

[CCycle]

Excellent thread. Many thanks to wwest and Brian for their insight.

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.