Quote:
Originally Posted by kannibul
(Post 460114)
Show me evidence that the only change is direct injection, and HP/Tq is gained.
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Of course DI involves other changes. When you further digitize the fuel delivery process, things like VVEL become even more efficient.
Wiki:
The major advantages of a GDI engine are increased
fuel efficiency and high
power output. In addition, the cooling effect of the injected fuel and the more evenly dispersed mixtures allow for more aggressive ignition timing curves. Emissions levels can also be more accurately controlled with the GDI system. The cited gains are achieved by the precise control over the amount of fuel and injection timings which are varied according to the load conditions. In addition, there are no throttling losses in some GDI engines, when compared to a conventional
fuel injected or
carbureted engine, which greatly improves efficiency, and reduces 'pumping losses' in engines without a throttle plate. Engine speed is controlled by the
engine control unit/engine management system (EMS), which regulates fuel injection function and ignition timing, instead of having a throttle plate which restricts the incoming air supply. Adding this function to the EMS requires considerable enhancement of its processing and memory, as direct injection plus the engine speed management must have very precise algorithms for good performance and driveability.
The engine management system continually chooses among three combustion modes: ultra
lean burn,
stoichiometric, and full power output. Each mode is characterized by the
air-fuel ratio. The
stoichiometric air-fuel ratio for
petrol (gasoline) is 14.7:1 by weight, but ultra lean mode can involve ratios as high as 65:1 (or even higher in some engines, for very limited periods). These mixtures are much leaner than in a conventional engine and reduce fuel consumption considerably.
- Ultra lean burn mode is used for light-load running conditions, at constant or reducing road speeds, where no acceleration is required. The fuel is not injected at the intake stroke but rather at the latter stages of the compression stroke, so that the small amount of air-fuel mixture is optimally placed near the spark plug. This stratified charge is surrounded mostly by air which keeps the fuel and the flame away from the cylinder walls for lowest emissions and heat losses. The combustion takes place in a toroidal (donut-shaped) cavity on the piston's surface.<sup class="Template-Fact" title="This claim needs references to reliable sources from March 2009" style="white-space: nowrap;">[citation needed]</sup> This technique enables the use of ultra-lean mixtures impossible with carburetors or conventional fuel injection.
- Stoichiometric mode is used for moderate load conditions. Fuel is injected during the intake stroke, creating a homogenous fuel-air mixture in the cylinder. From the stoichiometric ratio, an optimum burn results in a clean exhaust emission, further cleaned by the catalytic converter.
- Full power mode is used for rapid acceleration and heavy loads (as when climbing a hill). The air-fuel mixture is homogenous and the ratio is slightly richer than stoichiometric, which helps prevent knock (pinging). The fuel is injected during the intake stroke.
Direct injection may also be accompanied by other engine technologies such as
variable valve timing (VVT) and tuned/multi path or
variable length intake manifolding (VLIM, or VIM).
Water injection or (more commonly)
exhaust gas recirculation (EGR) may help reduce the high
nitrogen oxides (NOx) emissions which can result from burning
ultra lean mixtures.
