For the most part, engines run on two operating principles:
1. Diesel cycle
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2. Otto cycle
Certainly the most common is the Otto cycle, or spark ignition engine, or gasoline engine. The Diesel cycle has certainly seen rejuvenation of late and will be the topic of another column.
The two engines types both utilize four strokes of each piston to complete one cycle of taking in air and fuel, compressing this air/fuel mixture to make it burn more efficiently, burning the mixture and finally exhausting the burned mixture. What follows is the identical pattern of:
? Intake
? Compression
? Power
? Exhaust
Mechanically, these two designs ? Otto and Diesel ? are identical in that they both use a piston with a connecting rod attached to an offset crankpin on a crankshaft ? exactly what you would find on a bicycle. Also identical is the distance that the piston travels up and down its cylinder during each of its strokes. This is where the Atkinson Cycle differs dramatically.
The Atkinson design uses the four strokes of the Otto cycle to complete the combustion process, but unlike the Otto cycle, Atkinson's late closing of the intake valves produces an effective compression stroke that is shorter than the expansion stroke. This results in improved efficiency of the engine, at the expense of low-end torque. Hyundai claims that a traction motor makes up for the lacking torque compared to the base Sonata, with a net result of improved overall power and responsiveness.
Hyundai also uses something they call ?Direct Hybrid Blue Drive,? which uses lithium polymer batteries. These are a lighter, simpler, more efficient battery design compared to those used by other hybrids, which allows Hyundai to put more electric power to the ground when other manufacturers switch off electric boost and rely solely on the internal combustion engine for propulsion. They?ve also shied away from using CVT (continuously variable transmission) engines and have chosen to stick with a typical six-speed automatic transmission. But in place of a torque converter, they have placed an electric drive motor/generator coupled to a high efficiency oil pump to work the transmission.
There?s a whack of other goodies that are typical fare in a hybrid vehicle, but the big difference, as you?ve noted, is the use of the Atkinson cycle and lithium polymer batteries.
You might have heard the term "Atkinson cycle" associated with hybrids. There, unorthodox valve timing of the gasoline engine trades away a little power for enhanced efficiency. It's just the thing for optimal interaction of a hybrid's gasoline engine and electric motor. In fact, the Toyota Prius, Ford Fusion, Chevrolet Tahoe and other hybrids claim Atkinson operation.
In fact, these are only simulated Atkinsons. The real Atkinson cycle displays differences in piston travel, the intake and compression strokes are actually shorter than the subsequent expansion and exhaust strokes.
This video displays a neat cutaway of Honda's experimental Extended-Expansion General Purpose Engine, a single-cylinder powerplant intended for portable generators, lawn mowers and the like. Key to its operation is a multiple linkage of the engine's connecting rod and crankshaft. Like the original Atkinson "Cycle," circa 1886, its intake and compression strokes are shorter than those of expansion and exhaust. Thus, as well, there's a difference in displacement: 135.2 cc for intake/compression; 203.6 cc for expansion/exhaust. Compression ratio is 8.5:1; expansion ratio, 12.3:1, and one result of this is a 20-percent improvement in fuel efficiency.
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