In a garden-variety gasoline engine with fuel injection, gasoline takes a moreroundabout route than it does with the direct injection method. This indirect approachcauses all manner of inefficiencies in burning the fuel，and it can result in a lot of usableenergy getting wasted-and you not getting the most for the money you spent at thepump.
In a direct injection engine, however, the fuel skips the waiting period it would haveto endure inside a standard engine and instead proceeds straight to the combustionchamber. This allows the fuel to burn more evenly and thoroughly. For the driver, thatcan translate to better mileage and greater power to the wheels.
Basics
For the lay individual，the labyrinth of hoses，wire harnesses，manifolds and tubing beneath a car hood might appear intimidating. But when it comes to the gasoline engine,just know this：It needs fuel，air (oxygen，to be precise) and spark in order to operate.
The two most critical differences between a direct injection engine and a standard gasoline engine are how they deliver fuel and how the fuel mixes with incoming air. These basic premises make a tremendous difference in an engine's overall efficiency.
Before we look inside the direct injection engine, let's view a quick second in the life of a standard gasoline engine（for a more complete look at the gasoline engine，checkoutHow Car Engines Work). First，the fuel travels via pump from the fuel tank, throughthe fuel line and into fuel injectors that are mounted into the engine. The injectors spraygasoline into the air intake manifold，where fuel and air mix together into a fine mist. Atprecisely timed intervals, intake valves open, corresponding to the different cylinders ofthe engine. As a cylinder's intake valve opens, a piston in that cylinder descends，suckingthe fuel-air mist from the air manifold above into the chamber below. As the pistonascends once more，it squeezes (compresses) the fuel-air mix until it is nearly nine times asdense as it was to begin with. Then, that cylinder's designated spark plug fires, ignitingthe chamber into a high-pressure，high-energy explosion. This little bang pushes thepiston back down with tremendous force, causing it to turn the crankshaft and ultimatelysend power to the wheels.
With a direct injection engine，however, the fuel gets to skip a step and add a bit ofefficiency. Instead of hanging out in the air intake manifold, fuel is squirted directly intothe combustion chamber. With an assist from modern engine management computers, thefuel gets burned right where it's needed, when it's needed
Direct injection engines literally give you more bang for your buck, for two main reasons. One，they use a“leaner" fuel-air mixture ratio. Second, the way the fuel disperses inside the chamber allows it to burn more efficiently. Let's take a quick look at each.
The ratio of air to fuel as it burns in an engine will have certain, predictable effects on engine performance, emissions of pollutants and fuel efficiency. When the amount of air in the mixture is high, compared to the amount of fuel, it's known as a "lean" mixture.When the reverse is the case, it's called a "rich" fuel mixture.
Direct injection engines use a mixture of 40 or more parts air to one part fuel，written as 40，1. That compares to a normal gasoline engine's mix of 14. 7，1. A leaner mixture allows fuel to be burned much more conservatively.
A second efficiency plus for direct injection engines is that they can burn their fuel more completely. The fuel can be squirted directly where the combustion chamber is hottest-in a gasoline engine that means it ends up close to the spark. With a traditional gasoline engine, the fuel air mixture disperses widely within the chamber, leaving a substantial amount unburned and therefore ineffective.
Do direct injection engines represent a radical departure from the known and accepted principles of internal combustion?
The short answer is "no.”To be sure，direct injection engines do use a few special bits and technical tricks：
A nifty piece of hardware called a fuel rail，to distribute fuel to the injectors；
Special programming for the engine management computer to handle the calculations of flow rate, fuel droplet size，emissions controls and other things you don't want to think about while driving；
Special catalytic convertors to handle direct injection engines' notoriously high oxides of nitrogen emissions (NO=).
The NO issue notwithstanding, gasoline direct injection engines get high marks in particular for their cleaner emissions. It's for this reason that numerous engine companies have toiled to build two-stroke versions of the gasoline direct injection engine. While four-strokes are found on most automobiles and street-legal motorcycles，two-strokes rule when it comes to off-road motorcycles，small boat and personal watercraft engines and many of the motorbikes that serve as primary transportation in developing nations.
点击加载更多评论>>