How Stuff Works: How gasoline direct-injection engines work
The internal combustion gasoline engine -- we have a love/hate relationship with it. It creates a lot of pollution. It is incredibly inefficient. It spews carbon into the atmosphere. It makes us dependent on crude oil, with negative side effects ...
The internal combustion gasoline engine -- we have a love/hate relationship with it. It creates a lot of pollution. It is incredibly inefficient. It spews carbon into the atmosphere. It makes us dependent on crude oil, with negative side effects like oil spills and trade imbalances.
Yet we are completely dependent on the internal combustion engine. Nearly every car on the road today in America has one. And they will be with us for some time to come. Have you ever wondered why?
There are four things about the internal combustion engine that make it the engine of choice in the vast majority of our cars. First and perhaps most importantly, it is reliable. A typical engine will last at least 100,000 miles if you take care of it, and it will do its job in the hottest deserts and the coldest winters. Second, relatively speaking, it doesn't weigh much for the power it produces. An electric motor and its battery pack weigh a lot more. The same can be said for the cost of the engine -- batteries are really expensive right now. And finally, you can "recharge" an internal combustion engine by filling the gas tank. That process only takes a couple of minutes. For an electric car it takes hours and hours to recharge, making it impractical for any kind of long road trip.
If we could invent cheap, light, fast-to-recharge batteries, the internal combustion engine would die. If we could invent fuel cells that were inexpensive and reliable, the internal combustion engine would die. But right now there is nothing that comes close to the internal combustion engine, even with all of its problems.
So car manufacturers are trying to improve the internal combustion engine -- a process that has been going on for a hundred years. And the latest incarnation is pretty remarkable. It is called gasoline direct injection, and it should make it possible to get cars up to 70 miles per gallon or so, even before adding a hybrid drive. Just about every major manufacturer will be releasing some type of gasoline direct injection technology in 2011.
If you look across the claims from all the different companies, there are several common threads in what they are doing.
The engines all use a new type of fuel injector that can spray fuel directly into the cylinder during the compression stroke. To do that, they all need an extremely high pressure fuel pump rated at something around 2,000 PSI. In current cars, the fuel injection technology being used is called port fuel injection, where the injector sprays fuel at low pressure into the intake manifold.
The direct injection process allows the fuel to evaporate in the cylinder and cool the air/fuel mixture. That helps avoid premature ignition, so these engines can increase the compression ratio. Mazda's new direct injection engine goes to a 14:1 compression ratio, which has never been seen before in a production gasoline engine. The normal high has been 12:1 or so, and that would require premium fuel.
Many of the engines are using multiple injector sprays per stroke. One spray occurs as the air starts flowing in on the intake stroke, to cool things down. The second occurs right before the spark plug fires. This creates a stratified charge for a better burn pattern using less fuel.
The high compression ratio allows more energy to be extracted from the gasoline. Think of it this way: when gasoline burns in a higher-compression environment, it gets to expand further, meaning that more work can be extracted from the expansion. The stratified charge allows less fuel to be injected while still getting a good ignition/burn pattern. It allows a much leaner mixture in the cylinder.
One of the key technologies making gasoline direct injection possible is new piezoelectric fuel injectors. They are extremely precise and extremely fast. They also provide an unexpected side-effect in some cases. With these injectors, it is possible to start a warm engine without using the starter motor. The computer can find a cylinder that has its piston in the right position, inject fuel and fire it to start the engine. This stop-start ability should make it much easier for car manufacturers to create cars that no longer need to idle at traffic lights, saving a lot of fuel (and eliminating lots of urban pollution) in stop-and-go traffic.