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Originally Posted by cRiPpLe_rOoStEr
So-called "Atkinson cycle", altough it's not the real Atkinson which actually uses an articulated crankshaft to provide a power stroke longer than the compression stroke.
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While some original mechanical systems are attributed to the various inventors, Otto, Diesel, Atkinson... The heat engine cycle names: Otto, Diesel, Atkinson, Rankin, Brayton each describe a particular thermodynamic cycle, named to honor an inventor engineer or scientist. You can see the wikipedia page for each.
Modern hybrid car engines are closer to Atkinson than Otto. But it isn't the valve grind per se. The intake valve cam is advanced and retarded by an oil pressure driven mechanism that is calibrated to allow the precombustion pressure to be optimized for the engine rpm. In other words, the cylinder is filled with the same air-fuel charge independent of engine speed. Just less than the charge that would knock. The mechanical expansion ratio is about 13:1 but the effective compression ratio is lower.
The key advantage is that for power levels from about 15% of maximum up to full power, the engine output is proportional to rpm, because over a wide band the engine is producing maximum torque - due to having the intake valves providing a nearly constant amount of charge. With this a car can be designed to run with the throttle wide open maximizing thermodynamic efficiency. Instead of reducing power by throttling the intake charge, you reduce rpm.
Lets say you have a car with a 200 hp engine, and you only need 40 hp to cruise (up a grade...)
With an Otto cycle engine, you find a gear ratio that puts the engine at a reasonable rpm and throttle the engine to half or lower of its potential (which reduce effective compression ratio - which reduces efficiency). With an Atkinson like cycle engine, the engine is at wide open throttle and you need to find the appropriate gear ratio for the engine rpm that delivers the required power. The engine rpm will be 1/5 of red line. You need a CVT but you get excellent efficiency.
In the Otto cycle car you can open the throttle to speed up. In the Atkinson cycle car you need to change gear ratios, first going to a higher ratio, the car slows down but the engine rpm climbs, then at the new higher rpm, which is producing more power, you slide the gear ratio up to the speed that uses all the power. You can see that having a big electric motor to give you a boost could allow the car to drive acceptably. So you only see full time Atkinson engines in hybrid cars.
-mort