How Direct Injection Engines Work

By: Akweli Parker  | 
Image Gallery: Engines Most major car manufacturers make or plan to soon make gasoline cars with direct injection engines, such as GM’s Ecotec engine. See more pictures of engines.
Image Gallery: Engines Most major car manufacturers make or plan to soon make gasoline cars with direct injection engines, such as GM’s Ecotec engine. See more pictures of engines.
© 2009 General Motors

What kind of engine will propel your next car or truck? If you've been toying around with the idea of buying a diesel for its fuel economy (try nearly 50 mpg for the Volkswagen Jetta TDI), well, you may not need to give up on the trusted and familiar gasoline engine just yet.

That's because one of the technologies that makes diesels so miserly on fuel is coming to gasoline engines, too. It's called direct injection, and it refers to how the fuel makes its way inside an engine's controlled-explosion room, better known as a combustion chamber.


In a garden-variety gasoline engine with fuel injection, gasoline takes a more roundabout route than it does with the direct injection method. This indirect approach causes all manner of inefficiencies in burning the fuel, and it can result in a lot of usable energy getting wasted -- and you not getting the most for the money you spent at the pump.

In a direct injection engine, however, the fuel skips the waiting period it would have to endure inside a standard engine and instead proceeds straight to the combustion chamber. This allows the fuel to burn more evenly and thoroughly. For the driver, that can translate to better mileage and greater power to the wheels.

In the past, direct injection posed too many technical hurdles to make it worthwhile for mass market gasoline automobiles. But with advances in technology and greater pressure to make cars run more cleanly and efficiently, it looks as if gasoline direct injection -- or GDI as it's referred to in industry lingo -- is here to stay. In fact, most of the major car manufacturers make or plan to soon introduce gasoline cars that take advantage of this fuel saving and performance enhancing system.

Read on learn about the nuts and bolts -- and pistons and valves -- of direct injection.

Direct Injection Basics

With direct injection engines, fuel is squirted directly into the combustion chamber instead of hanging out in the air intake manifold.
With direct injection engines, fuel is squirted directly into the combustion chamber instead of hanging out in the air intake manifold.
© 2009 HowStuffWorks

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, check out How Car Engines Work). First, the fuel travels via pump from the fuel tank, through the fuel line and into fuel injectors that are mounted into the engine. The injectors spray gasoline into the air intake manifold, where fuel and air mix together into a fine mist. At precisely timed intervals, intake valves open, corresponding to the different cylinders of the engine. As a cylinder's intake valve opens, a piston in that cylinder descends, sucking the fuel-air mist from the air manifold above into the chamber below. As the piston ascends once more, it squeezes (compresses) the fuel-air mix until it is nearly nine times as dense as it was to begin with. Then, that cylinder's designated spark plug fires, igniting the chamber into a high-pressure, high-energy explosion. This little bang pushes the piston back down with tremendous force, causing it to turn the crankshaft and ultimately send power to the wheels.

Got that? Pretty complicated, huh? It works, but from an engineering standpoint, it leaves much to be desired, and it's pretty wasteful.

With a direct injection engine, however, the fuel gets to skip a step and add a bit of efficiency. Instead of hanging out in the air intake manifold, fuel is squirted directly into the combustion chamber. With an assist from modern engine management computers, the fuel gets burned right where it's needed, when it's needed [source:].

To find out more about what makes direct injection engines more efficient, move on to the next page.

Direct Injection Engine Efficiency

A 2010 Ecotec 2.4L engine fuel rail. This part distributes fuel to the injectors.
A 2010 Ecotec 2.4L engine fuel rail. This part distributes fuel to the injectors.
© 2009 General Motors

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.

So what about the rest of the engine? 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 (NOx)

The NOx 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.

In the next section, we'll examine why squirting fuel directly into the combustion chamber under high pressure doesn't result in blowing up the engine.

Direct Injection Engine Safety and Reliability

In 1893, Rudolf Diesel determined to prove his theory that a fuel could be made to burn by subjecting it to extreme pressure, without an ignition source. Furthermore, he demonstrated, the resulting energy could be transferred to a machine for the purpose of performing work. Diesel nearly killed himself perfecting his method, which is known as compression combustion. But it ultimately made him a millionaire [source: Energy Information Administration].

The diesel engine evolved in the 1920s to incorporate direct injection as a hallmark of its design. Diesels are built quite sturdily to survive the high stresses generated when they operate.


Gasoline, on the other hand, poses multiple challenges for anyone attempting to burn it in a direct injection application. For one thing, the injectors themselves must be able to withstand the extreme heat of the combustion chamber. And because of the different burn characteristics of gasoline, detonation was a problem. ("Detonation" is just another term for knocking, or multiple flame fronts colliding in a combustion chamber, which can cause serious harm to an engine.) Another problem that plagued GDI engineers was "coking," the build-up of cooked fuel deposits that fouled the injectors [source: Noyes, Wells].

Consequently, one of the biggest delays in getting GDI-equipped cars to the masses has been the fine-tuning research and development needed to make them as reliable as their standard counterparts. Automotive component suppliers including Bosch, Delphi, Denso and Siemens have come up with solutions: Rugged, high-pressure fuel injectors and the sophisticated electronics to make them perform optimally in GDI engines.

In the next section, we'll take a closer look at the pieces of a direct injection fuel system.

Components of a Direct Injection Fuel System

You'll find a couple of items in every direct injection engine: fuel injectors (at least one per cylinder); and a combustion chamber -- the gaseous hothouse that comprises the upper portion of every cylinder.

Additionally, depending on what type of direct injection system it belongs to, the engine may or may not feature several other components affiliated with the following systems:


  • Common Rail System -- A long metal cylinder called a fuel rail distributes fuel to the injectors under extremely high pressure.
  • Distributor and Inline Pump System -- Either a rotary wheel distributor or plunger-style pump is used to push pressurized fuel to the injectors.
  • Unit Direct System -- In this setup, the injector and a fuel pump just for that injector are integrated into a single unit and positioned over each cylinder [source: Bosch].

One more distinction between direct injection and regular engines: With a direct injection engine, the portion of the car's computer responsible for fuel management has to think a lot more quickly. That's because the fuel management system must pulse fuel into the cylinders at much shorter intervals. And in general, the precise rationing of fuel and manipulation of air-fuel ratios is more crucial on a direct injection engine in order to optimize performance, emissions and fuel efficiency [source: Sawyer].

Read on to learn some of the possible advantages of driving a car that packs a direct injection engine -- along with some reasons they took so long to catch on.

Pros and Cons of Direct Injection Engines

Like clothing styles, the notion of reaping benefits from gasoline direct injection engines seems to come and go and come back again. They're alluring to auto engineers for a couple of reasons: the potential savings from fuel efficiency and the potential performance boost, especially if used in concert with a turbocharger. For example, Bosch -- a German supplier of high-tech auto components -- says its gasoline direct injection system cuts fuel consumption by 15 percent and can provide up to 50 percent more low-end torque than a comparable indirect injection system [source: Bosch].

So what's the catch? Well, GDI engines tend to boast clean emissions profiles overall, but they do produce excess oxides of nitrogen, or NOx. Like diesels, they too can spew unsightly and unhealthy particulate matter -- that's environmentalist-speak for "soot" [source: Sawyer]. And for the time being, at least, they cost more to build than ordinary engines. Audi engine development chief Axel Eiser estimated the direct injection engines being put into Audis will cost 5 percent more to build than an ordinary engine. Other estimates say a direct injection engine could cost several hundred dollars more than its indirect counterpart, owing to its more complicated emissions controls [source: Csere].


But as is usually the case with technologies new to the marketplace, developers have been steadily ironing out the problems. The NOx issues have all but disappeared by using a technique called exhaust gas recirculation (EGR). Catalytic converters specifically formulated for GDI engines cut down even further on pollutants [source: Visnic].

With consumer concern about the environment and gasoline prices approaching critical mass, automakers are ramping up production of their GDI offerings and preparing to launch new ones. Move on to the next page to see what automakers have in mind for the future of this reborn technology.

Direct Injection Engine Vehicle Makers

A visitor examines the new Mazda Motor Corp.'s direct injection engine "Mazda SKY-G" during the Tokyo Motor Show in October 2009.
A visitor examines the new Mazda Motor Corp.'s direct injection engine "Mazda SKY-G" during the Tokyo Motor Show in October 2009.
Kiyoshi Ota /Getty Images News/Getty Images

As noted earlier, cars that run on diesel fuel use direct injection by default. For a multitude of reasons, however, diesel hasn't captured the hearts and car-buying dollars of the American driving public in large numbers. It succeeded more readily in Europe, where fuel-efficiency may trump other concerns, such as noise and acceleration. (Diesels were once louder, slower and dirtier than gasoline cars.)

It should come as no surprise, then, that European automakers have taken the lead in developing gasoline direct injection technology. However, Japanese automaker Mitsubishi gets to claim "first" honors for making a GDI for a mass-market consumer vehicle. Mitsubishi introduced one in its Galant in 1996 for sale within the Japanese market [source:]. Toyota soon followed suit, along with European automakers. Initially, though, the engines had difficulty living up to their promise of significant fuel savings.


U.S. manufacturers were aware of GDI at that time, but didn't aggressively pursue it until several years later. This may have been due to the fact that, in the late 1990s and early 2000s, oil and the gasoline derived from it were relatively cheap, which made the need for offering fuel-saving engines such as GDIs a little less urgent.

That has changed drastically, with Ford and GM now trumpeting the technological innovations to be found in their GDI designs. GM's Ecotec direct injection engine family will be a growing staple in its car and SUV lineup, with more than a dozen models slated to get Ecotec powerplants by the 2010 model year [source: Green Car Congress].

Also, Ford announced that more than 2.5 million vehicles will get its "EcoBoost" gasoline direct injection engine between 2009 and 2013. The company plans to put them in its Lincoln MKS, Ford Flex crossover and Taurus and claims that drivers may see up to a 20-percent increase in fuel efficiency [source: Ford]. Other manufacturers offering or planning to offer GDI engines include Audi, BMW, Hyundai, Kia, Mazda, Mercedes-Benz, Nissan, Lexus, Saab, Subaru and Volkswagen.

The big unknown is whether gasoline direct injection will get leapfrogged by more advanced technologies. Hybrid gasoline and electric engines have also captured the public's imagination. Plug-in systems that allow drivers to recharge car batteries from a home electrical outlet could eventually become widespread, too. And fuel cell-powered hydrogen vehicles may not be as far-off in the future as people once thought, as evidenced by the Honda FCX.

But most likely, automotive analysts say, direct injection will help serve as an evolutionary bridge to those more exotic options. So if you can't wait for the clean, silent "car of the future" but still want to save a little pocket change at the pump, a direct injection-equipped ride might be the ticket. For more information on engine technology, check out the links below.

Originally Published: Dec 2, 2009

Lots More Information

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More Great Links

  • Avery, Noyes L. and Wells, Paul Patrick. "US Patent 6746495 - Method for controlling deposit formation in gasoline direct injection engine by use of a fuel having particular compositional characteristics." Issued: June 8, 2004. (Accessed on July 25, 2008)
  • Bell, Corky. "Maximum Boost -- Designing, Testing and Installing Turbocharger Systems." Bentley Publishers. 1997.
  • Birch, Stuart. "Tech Briefs: Diesel vs. Gasoline, The Fight Goes On." Automotive Engineering International Online. January 2000. (Accessed on July 27, 2008)
  • Csere, Csaba. "Will Gasoline Direct Injection Finally Make It?" Car and Driver. June 2004. (Accessed on July 22, 2008)
  • Energy Information Administration. "Rudolf Diesel." Accessed November 20, 2009.
  • European Aeronautic Defense and Space Company N.V. "History of Aviation -- Messerschmitt Bf 109." (Accessed July 27, 2008)
  • Ford Motor Co. "Ford to Equip Half a Million Vehicles With EcoBoost Engine." Ford Motor Co. press release. Jan. 6, 2008. (Accessed July 20, 2008)
  • Green Car Congress. "GM's Expanding Gasoline Direct Injection Engine Portfolio." April 13, 2009. (Accessed November 20, 2009)
  • General Motors. "New Ecotec 2.0-Liter Direct Injection Turbo Engine." General Motors Press Release. March 31, 2006. (Accessed July 19, 2008)
  • Editorial Team. "Research Analysis: A Review of Gasoline Direct Injection Systems." December 7, 2007. (Accessed July 21, 2008)
  • Mitsubishi Motors. "GDI1 Engine Production Tops 1,000,000 Unit Mark." Mitsubishi Motors press release. Sept. 11, 2001. (Accessed on July 23, 2008)
  • Robert Bosch GmBh. "Direct Gasoline Injection." (Accessed July 22, 2008)
  • Robert Bosch GmBh. "Innovations yesterday and today." (Accessed July 22, 2008)
  • Sawyer, Christopher A. "Directly Speaking About Direct Injection." Field Guide to Automotive Technology. August 2005. (Accessed July 24, 2008)
  • Stuart, Herbert Akroyd. "US Patent 502837 -- Engine Operated by the Explosion of Mixtures of Hydrocarbons and Air." Issued August 8, 1893. (Accessed August 1, 2008)
  • TIME Magazine. "Detroit's Total Revolution." March 19, 1979. (Accessed July 21, 2008),9171,947023-1,00.html
  • TIME Magazine. "Diesel Dazzle." Nov. 15, 1976. (Accessed July 20, 2008),9171,712317,00.html?promoid=google
  • Visnic, Bill. "Direct Injection Coming for The Masses." Ward's Auto World. September 1, 1996. (Accessed July 19, 2008)
  • Visteon Corporation. "Powertrain Controls -- Low Pressure Direct Injection Fuel System." (Accessed July 19, 2008)
  • Witzenberg, Gary. "The Story Behind Ward's 10 Best Engines." Ward's Auto World. April 1, 2008. (Accessed July 21, 2008)