The Perfect Compression Ratio
The compression ratio of your combustion engine – around 14:1 is the optimum for performance and economy, although it's usually lower to prevent knock at high temperatures.There's another huge compromise here. The compression of a piston engine might be 14:1 when the throttle is wide open and the cylinder is being completely filled with air and fuel, but whenever you close it – even a fraction – you'll start to create a partial vacuum inside the cylinder. It simply won't be able to fill during the intake stroke, so there's less mixture to compress during the compression stroke. That means that when you're not on full throttle, the engine's compression ratio is lower – there isn't as much air and fuel being squeezed.
On turbo or supercharged engines, it's even worse: because mixture is being forced in under pressure at full throttle, the actual compression ratio much be lower to make sure it doesn't detonate, typically nearer 8:1. But at lower speed, when the turbo isn't blowing, you get hit with a double-whammy: not enough mixture to fill the cylinder, and an intentionally low compression ratio. The result is a serious lack of power – hence the 'flat' feeling of turbo engines when they're off-boost.
Again, this is something a lot of manufacturers are looking at. There are several solutions, from moving plugs inside the combustion chamber to alter compression, as on Lotus's Omnivore two-stroke design, pistons with crowns that can be raised or lowered hydraulically, which is Honda's preferred system, or even systems that alter the height of the entire piston, as with the well-publicized MCE-5 prototype engine. The problems all face are in the mechanical complexity of the systems and the difficulty of keeping a neat, well-shaped combustion chamber.
Prototype engines equipped with the systems, normally with turbos attached to show off to the best of their abilities, have, however, been impressive, exhibiting incredible performance and fuel economy improvements. We sure we will see this technology soon on our favorite two-wheeler
