The EPA calculates automobile efficiency by using standard driving routes. The urban route measures city driving MPG and a highway route measures highway MPG. The urban driving route is mostly stop-and-go. The power required for this trip is shown in the first figure. The right side of the figure shows the engine’s efficiency map. The best efficiency is near the top of the map. When the engine’s power for city driving is plotted on the map, it can be seen that the engine is usually inefficient.
Hybrids solve this problem by operating the engine at or near it most efficient point. The power mismatch is corrected with batteries and electric motors. When extra power is needed, it comes from the batteries. When the gas engine can efficiently produce extra power, it goes into the batteries. The Prius engine is not more efficient, it just runs when it is efficient. The hybrid does not change or move the efficiency point of the engine. Adding batteries and electric motors allows the automobile to provide the needed power efficiently.
In the second figure, the Prius efficiency map is shown. The most efficient area is close to peak power. Because the Prius has electric motors for traction power, the gas engine is smaller. The smaller engine offsets some of the weight penalty of electric motors and a traction battery. Naturally, there are some inefficiencies in putting power into and getting power out of the batteries. But hybrid technology allows the benefits of regenerative braking and start/stop engine behavior.
The YankeeDiesel is more efficient than traditional diesels because the combustion gases expand a second time inside the engine. That efficiency gain is estimated at 4% at peak power. But the YankeeDiesel has a throttle that effectively changes the engine size and its peak efficiency is over a broad power range. The 4% improvement increases to 17% at lower power. This is shown in the third figure.
Traditional diesels do not have throttles. They take in the same amount of air on each stroke and vary the power by controlling the fuel flow. At low power there is more air in the combustion chamber than needed. That extra air does not contribute to the combustion process. It goes along for the ride, gets heated up and is thrown out with the exhaust. That is wasted energy. The YankeeDiesel solves that problem by varying the size of the combustion chamber. Now the ideal air/fuel mixture is extended to lower power levels.
Next the YankeeDiesel’s thermodynamic process is compared to a traditional diesel. The pressure/volume curves are identical for the compression and combustion strokes. But near the end of the power stroke they differ. The YankeeDiesel allows the combustion gases to expand a second time. The combustion chamber pressure drops as the secondary expansion fills. The combustion gases are very hot and mix with the air trapped in the secondary expansion chamber. That mixing converts some of the thermal energy into extra pressure. That pressure then expands to produce more power.
At the bottom of the figure, the YankeeDiesel’s temperature/entropy curve is compared to a traditional diesel. As before, the compression and combustion processes are the same. Near the end of the power stroke, the YankeeDiesel temperature drops more. The temperature drops because its thermal energy is converted into extra pressure in the secondary expansion chamber. Hence the exhaust gases are at a lower temperature since more energy was converted to mechanical power.