Hi, I-IMA!
Thank you for directing us to the interesting sites. The fact the 400h averages 25mpg(U.S.) is in itself amazing. Just imagine saving total gas consumption by 10 percent in the entire world to-day without forcing drivers switch to smaller vehicles. It is very possible technically.
Why hybrids show better mileage even during highway driving compared to similar gas only automobiles in spite of the fact that they have to carry around heavy batteries? Before getting the answers, we have to know the following fact:
When cruising at a highway speed, automobiles require only a fraction of horsepower their engines are capable of. This means you are driving with the gas pedal halfway to the metal feeding a small amount of air and fuel.
100HP, 150HP, 200HP or even 300HP of power is needed when starting, passing, going uphill, towing a trailer or speeding flat out. Now, how do you get this, in many cases sudden, requirement of more power? In the case with gas engine automobiles you step on it to burn more gas.
In the case with most hybrid automobiles the electric motor provides this additional power requirement, at least partially. This means in a hybrid automobile you can have a gas engine specs of which are tailored with this electric motor assistance in mind.
Therefore,
1. Honda uses cylinder deactivation when cruising at a highway speed.
2. Both Honda and Toyota use high-efficiency Atkins cycle engine, which in effect has displacement smaller than it looks by making the effective compression stroke shorter than the expansion stroke, thus allowing the hot, expanding gas inside the cylinder to do more work.
Smaller displacement means the gas engine must work harder even when cruising, requiring wider opening of throttle valve, which in turn reduces pumping loss making the engine more efficient. Also, a good portion of power stored in the electric motor batteries comes from the energy recovered when coasting and braking, which ordinary gas engine automobiles simply throw away.
If you can do away with the throttle valve (butterfly valve) altogether, the gas engine would become more efficient. Easiest way to accomplish this is the lean burn design. But the cat converter does not like unused oxygen in the exhaust gas. So, it looks like the best approach is to control opening and closing timing of the intake valves according to the gas pedal position and movement, thus accomplishing reduction of pumping loss and at the same time changing of effective length of compression stroke. From this standpoint BMW’s Valvetronics technology looks very promising. Now, when will BMW bring out its own version of hybrid?
