ICE efficiency of Prius vs Insight

Some of us with Graham scanners have been doing this for a while.

Now I've got two thick reports to read at dinner! <GRINS>

Thanks!!! You can always let me know about recent work like this. It is my 'meat and bread.'

Bob Wilson

 

Oh I see the why now, thanks for the detail explainations.

 

Further to what Pat said, the more air in the cylinder, the more energy is lost to compressing it and to friction.

If you allow more air in you will end up with higher pressure, which will increase the forces on the piston, which will increase the friction to the cylinder walls (the rings are forced against the walls by gas pressure mostly, although there is a small amount of spring pressure).

Further, the losses increase due to the higher number of air molecules (mostly N2 and secondly O2). Each one can be considered an object that will be heated by compression. The more you heat, the more energy is used. You do recover -some- of this energy when the piston moves down, but the losses are proportional to the number of molecules. The energy goes into the block/coolant system.

The extreme low value is if you had a vacuum in there. Then the loss would be from ring spring pressure against the cylinder walls and bearing friction losses in the connecting rod ends and crankshaft.

When I used to mess with engines I found a well constructed engine could be spun slowly with one finger (spark plugs removed). Put the spark plugs in and it took at least a hand, perhaps two hands. Valve mechanisms disconnected, of course. High rate spinning by hand was just not possible.

In my power mechanics courses we were taught internal combustion engines ran at between 20 and 25% efficiency. But this was in the late 1960s/early 1970s. They have improved a bit since then to about 25-30% with better/more accurate fueling systems. So the Prius engine is phenomenal!

 

Without compression you have pumping losses. Close the valves and you get rid of the pumping losses, but you increase the frictional losses with the rings. It's a trade off.

Tom

 

I think the frictional losses still exist even on valve openings.

Anyway, following chart by Honda's study shows 66% energy loss reduction on valve closing came from following page (Japanese only).
CIVIC/CIVIC HYBRID

Ken@Japan

 

The only risk I see is if there is ever a negative pressure in any of the strokes. This might tend to draw oil into the cylinder and potentially lead to problems with the spark plug. But if the intake stroke occurs first so a positive pressure is always maintained, a non-combustion stroke should be like compressing an air-spring. The energy needed to compress the gas would come from another cylinder expanding the cold gas. Non-combustion pressures are relatively light.

Of course no losses would be even better.

Bob Wilson

 

They do, but pressure should seat the rings a bit more. I imagine the difference is small as compared to pumping losses.

Tom

 

Surely you mean that 28% of the engine TORQUE is transmitted to MG1. The amount of power will vary with MG1 speed and could be zero if MG1 is stationery as will occur over a range of speeds and road loads.

The power is even negative under some conditions.

kevin

 

That is correct. The torque split is constant, the power varies with speed. That is why Toyota calls it a PSD, or Power Splitting Device.

Tom

 

back to the original post. Honda need to make the engine in the Insight spin freely under no load because the engine spins all the time the car is moving. With HSD the engine does not spin under deceleration and may actually be being used as a brake to allow MG1 to generate electricity. In Honda's systen the engine must spin to allow the motor/generator to spin and make electricity. Because Honda want to make as much electric as possible they make the engine spin freely. With HSD there would be much less benefit in making the engine spin freely but there is some benefit and for this reason Toyota use variable valve timing to reduce the compression losses by allowing some of the air in the cylinder to go back into the inlet manifold. The variable valve timing also allows for the short compression stroke, long power stroke that results in very high efficiency at part load.