Gasoline engines burn fuel stoiciometrically (proper amount of fuel for the oxygen in the charge) and use the momentum of the exiting burned charge to pull a new charge in. This requires that the intake valve open a little before the exhaust valve closes. This is called "valve overlap". At this time the exhaust gases are rapidly flowing out the exhaust valve. The momentum of the gas forms a vacuum in the cylinder, which is filled by the intake charge. The Peugeot brothers invented this, and came over from France and beat the pants off the locals at the Indy 500 to prove it worked. Ever since then automobile engines have had valve overlap. This is more efficient, because just the energy that would be wasted in the exhaust is doing work. Prior to the Peugeot brothers, cars revved not much much over 1 or 2 thousand RPM. Afterwards, the state of the art is about 18 K RPM in a F1 race car.

To control the charge energy (number of molecules of air and gas in the cylinder) during the burn gasoline cars have a throttle to cause a partial vacuum at the intake valve. This vacuum takes energy to overcome. If more air was fed in than the amount of fuel could burn, the mixture would explode (called detonation in car talk).

Additionally, due to the aerodynamic action of valve overlap, there are optimum RPM's and engine loads for the best cylinder scavenging, and charging action. I am not sure of the numbers but for standard engines, I believe best efficiency occurs at .6 rated horsepower at about .8 max rpm. Car enthusiasts feel this little extra push and call it "coming on cam". The cam is the device in the engine that sets valve overlap (unless the engine is a VVT). This is where the engine will generate the best energy out for the fuel consumed ratio.

Now, holding almost any regular car at .6 rated horsepower for very long, and you will be doing 95 mph. While the engine is at its most efficient, the car system is not. Because the car aerodynamic drag force goes up by the square of speed, and power required by the cube of speed.

The reason standard cars have the power they do is for reasonable acceleration. If the car only had enough power for the typical max speed, say 85 mph, then it would take 3 or 4 times as long to accelerate. Examples of vehicles setup for best typical speed efficiency are trains and 18 wheel truck. Those vehicles have enough power to maintain maximum speed, but dramatic gearing is required for any acceleration at all.

The problem is that a car at the typical average speed (50 mph - 35 for local roads and 65 for highway) the engine is operating well below the best efficiency point. Standard car engine efficiency can be half (about 12 percent) of what it would be at optimum power/rpm (about 25 percent). Yikes!! Engineers call this the "Partial Power Problem".

This is actually the technical foundation of the Hybrid concept. By sizing the engine closer to the average power requirement, and using the battery/motor for the power peaks, better acceleration efficiency is obtained as the engine is running closer to its peak efficiency point. Additionally after acceleration the engine can run at a higher efficiency point while only a small amount of power is needed to push the car, as the batteries need to be recharged. And lastly, regeneration can be used for braking the car, and that recovers some of the energy put out by the engine to get the car to speed.

As an aside, in standard cars it ismore difficult to use an efficient level of acceleration due to fuel system "enrichment". Both carburetor and fuel injection cars will push a little extra squirt of gas on acceleration. This is called "enrichment". Its done to avoid detonation, I believe. This is why people are told to accelerate slowly for best mileage in standard cars, too. Actually, its the rate of acceleration, or how quickly you push the pedal, not what the pedal ends up at that is important for efficiency. This is probably partially responsible for the Prius little old lady - poor MPG phenomena.

The Prius relies on the battery for rapid start of acceleration function, so as I understand it, they left enrichment out of the engine fuel system. The battery comes up quickly, but the engine comes on load more gradually than a standard car.

Additionally, the Prius engine has a special way to improve engine partial power efficiency and best efficiency. At about 1/3 power the Prius engine is already near its maximum (30%) efficiency. So, the Pulse needs to get the engine power up to about 1/3 to 1/2 the engine power until one reaches a speed where the power to overcome aerodynamic drag becomes wasteful. Remember, that is a very sharp function (cubic). During the pulse the CVT does the speeding up, while the engine comes up to a high efficiency point.

At the average of the pulse/glide (35 mph), the power required is well below the 1/3 engine power even for the smaller Prius Engine (76/3 = 25.3 hp) . The 28 hp battery can even push the car along (not for long) at 55 mph, including cranking the unfueled engine over. At 35 mph, power needed is probably under 10 hp.

- Donee (PriusChat, with some mild modifications by Darell)