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]

No "room" for batteries, etc, in an airplane, LSA, so trying to find a way to run a small engine in two modes efficienctly, BIG and little.

~2500 RPM limitation.

 

[CCycle]

You do not need batteries for the differential/electric counter method. That is what the second motor is for. But still we must agree that an aeronautical application is not ideal for this technique.

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.

 

[wwest]

One of the problems I have with turbocharging is with the method used to moderate the boost level, the wastegate.

Wasting energy at the very time it is most needed.

With a variable speed/volume SC no "wastegate" equivalent would be needed.

 

[BrianPetersen]

Variable-geometry turbochargers don't have that issue. Every VW TDI engine since 1999 has one.

 

[TDIMeister]

The wastegate is almost never open during part load. It's only there to prevent overspeeding and overboosting of the turbo during high load and to regulate boost during transients. I don't see how energy is "wasted" in a wastegate compared to a S/C if the energy driving the turbo is itself "waste" (i.e. exhaust) energy that would otherwise go to the tailpipe anyway.

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.