The primary advantage of an Atkinson cycle engine is that MORE of the A/F mixture is burned during the power stroke, INSIDE THE CYLINDER, resulting in LESS heat/energy entering the exhaust system when compared to an "otto" engine.

 

My understanding is that combustion is essentially complete in the combustion chamber of any modern ICE. Exhaust flow/pressure is just dependent on the volume of air/gases exhausted from the engine which, although probably less in an Atkinson cycle, are certainly enough to make the exhaust flow out of the system and spin a turbo. As I said, not as effectively as a conventional engine but it would run. The most significant advantage of the Atkinson cycle is the asymmetry of the cycle where less energy is required in the compression stroke, providing more usable power during the power stroke since less is used during compression. Other than this effect, the Atkinson cycle would not be much more efficient than modern lean burn engines.

 

Just did a quick Google. Suburu has built a prototype hybrid with an Atkinson/Miller cycle turbocharged engine:

http://www.greencarcongress.com/2006..._introduc.html

 

Be that as it may, we all know that the bane of a turbocharged engine is the fact that you can't spin the turbine and produce BOOST unless a significant level of the product of combustion reaches the exhaust "stack". In an Atkinson cycle engine more of the "product" of combustion is used to push the piston downward, as opposed to the "otto" engine.

I'm somewhat surprised, obviously, to see the Subaru announcement. But the use of the electrics to take up the "slack" in HP/torque until the turbocharger begins producing boost is undoubtedly a good idea. It will be interesting to see how they overcome several of the other problems concerned with converting the Atkinson cycle technique into the Miller cycle using a turbocharged engine.

With the RDX, Acura has just learned, is learning, a hard lesson, if you're going to use a turbocharger to provide boost LATE in the "cycle" then you must derate the engine (low "native" compression) during normal "off-boost" running. Since the latter is the clear majority of the time, absent being on a race track, the FE suffers mightily.

The second issue with turbocharging a hybrid is the problem of keeping the catalyst heated to the operational level. But it looks as if Subaru is planning for their ICE to run continuously so that might not be a problem for them.

I guess we'll alll...

Stay tuned.

 

,,, and, because of the power assist availablity of the hybrid to help with small changes in power demand, the hybrid tends to deliver power within a narrower RPM range over the same road -- and that RPM range is in the optimum efficiency range for the engine.

 

I think the advantage you credit to the hybrid power assist should be more correctly attributed to the CVT. The CVT, unquestionably, allows the ICE to operate the clear majority of the time (no acceleration component) just barely above the point of knock/ping due to engine lugging.

There are two optimum efficiency range "bands" for the ICE, the lower RPM "band" resulting in the lowest possible frictional component but with relatively HUGE pumping losses, and the higher power output "band" where the opposite becomes true, pumping losses are minimized and frictional components go through the roof.

I personally think the power assist capability is best described as a form of supercharging, a HIGHLY efficient form of supercharging, allowing the use of a relatively small and therefore highly efficient ICE. ANYTIME the power requirement is above that of the smallish but HIGHLY efficient ICE the hybrid power assist will take up the "slack".

Excluding, of course, those times the ICE is not running at all.

 

I don't think supercharger is right; a supercharger uses a mechanical linkage that functions exactly the same at all RPM (increasing/decreasing with speed). The eCVT battery is much less predictable.

 

No, I was referring specifically to the more minor efficiency wherein the ECU does not change the ICE drive ratio becasue it simply does not need to, like a small hump or minor grade change that is temporary. It can handle the small difference in torque demand by simply using the power assist, so there is no need to change the engine speed. This, the ICE operates in a near constant RPM range, and is more efficient as a result.

If you drive an HSD hybrid, you have seen the assist come on for brief periods while in cruise and then go right back off. This serves two purposes: 1> It provides an opportunity for the ECU to use some of the stored energy if the battery is in a high SOC (so that any future regen energy has somewhere to go); and 2> it permits the ECU to manipulate the electric assist to maintain a near-constant ICE RPM (an ECU objective becasue its more efficient that way).

Conversely, when on a very slight downgrade (not enough for coasting), placing the MG system in regen has the effect of decreasing the (negative) torque demand on the ICE (a reduction in torque demand). If the MG load (either as motor or generator) is managed to as to handle the slightly decreased axle torque on the downgrade, no ICE RPM change is necessary. Once again it is more efficient realtive to whatever power it is producing.

This does not hold true for extended or steeper grades or coasting situations. There the CVT is the primary factor.

A hybrid is more that a CVT and an hybrid-adapted engine. It contains lots of little efficiencies that when combined make a big difference.

In this case, the Angel is in the details.

 

The main benefit of the "Atkinson"-cycle ICE, as I see it, is that by making the expansion stroke longer than the effective compression stroke, a higher percentage of the heat energy of the burned gases is being extracted during the power stroke than is the case in a standard Otto-cycle ICE. The engine is thus more thermodynamically efficient. The exhaust gases are consequently cooler (less wasted heat energy) than they would have been in a conventional ICE. But, the effective compression ratio of the fuel-air mixture (and so the work done compressing it) is still ~10:1 in an Atkinson-cycle ICE, just like a conventional ICE running on regular gasoline — it's the expansion ratio that is much larger (~13:1).

Stan