Originally Posted By: LT4 Vette
Whats so special about DI ?
High injection pressures and spraying directly into the combustion chamber means that the latent heat of evaporation can be used to cool the charge in the cylinder. This reduces the chance of detonation and knock and allows the manufacturer to use a higher static compression ratio which in turn increases the thermal efficiency of the combustion process. This is why diesels are considered so efficient because they can extract more torque from an equal amount of air/fuel (and burn it more efficiently) due to their high compression ratio. Other inherent benefits are more precise control of fuel vapourization, ignition event timing and rate ie. injecting many 'sprays' of fuel per injection cycle AT the time of combustion (near TDC, TDC and just after TDC if needed).
The ONLY downsides are fuel spray adhering to the cylinder wall, which may be skimmed over by the ring pack and see lubricating oil (dilution), intake port deposits from lack of port washing by fuel and slightly increased cost to implement. Progress is being made in all three areas though. Differently angled injectors and different pressures and injection cycles helps prevent fuel adhesion to the wall, better oils, ventilation systems and EGR methods are helping the port buildups and ... well more massive implementation of DI systems is driving component cost down.
Another benefit of DI is that, combined with precise engine control and ignition, the engine can be stopped and restarted completely by combustion without the use of motors or servos. This means restart times are reduced to a fraction of what's available now in hybrids etc. FWIW, it's really interesting how it works. The engine keeps track of the crank angle very intimately, and stops the engine so that pistons are all level, halfway down (or up) the bore. It uses the alternator as a loading device to get the engine to stop at exactly this position. Once restart is required, the DI system injects a small amount of fuel into a cylinder that was on it's way up, and ignites it causing a small combustion event pushing the engine backwards a few degrees. Right as this happens, another cylinder that was halfway down it's power stroke travels backwards, up to TDC building compression at which time DI injects fuel into that cylinder and preignites it driving that cylinder down the correct way and starting the engine. All of this happens within 0.35 seconds.