What is direct injection?

direct injection

Direct injection at work: The fuel injector, at left sprays a fine atomized mist of fuel directly into the combustion chamber, illustrated here in a Ford EcoBoost engine.

A new automotive buzzword is “direct injection.” Not to worry, it doesn’t have any medical implications, but despite having a performance enhancing effect, it’s approved, encouraged even, for automotive use. Understanding why can take us back, if not to the beginning of time, to the 19th century:

An engine need a mixture of air and vaporized fuel in a ratio of about 14:1 to run properly or indeed to run at all, and the device used almost universally to make this air-fuel mix from the beginning of the gasoline internal combustion engine—Karl Benz patented it in 1886—had been the carburetor. Essentially this is a tube with a pipe sticking into it and the low pressure area caused by the airstream rushing through the tube sucks gasoline in the tube out into the air, where the turbulence vaporizes it in the process. It’s more complicated than that, of course, and over 80 years of development it became a rather sophisticated instrument. But it is still what some wag called a “controlled leak.”

Fuel injection—squirting a mist of fuel into the engine’s intake tract—was the next step, but early examples, such as the fuel injected Chevrolet engines of the Sixties, were expensive and required more attention than did the carbureted engines of the day. As a result, fuel injection was limited to extra-high performance applications, such as the “fuelie Corvettes.”

These early mechanical-pump injection systems weren’t particularly fuel efficient nor were they terribly compatible with the first emission requirements that began in 1967. But electronically-controlled fuel injection started to become available in the early seventies, most notably from Volvo. With its box of transistors, however, it was expensive, so most carmakers stayed with the tried and true carburetor.

But increasingly stringent emissions requirements extremely compromised the carburetor, and by the early eighties, the typical carb had so many tubes and connectors that made the engine look like it was on life support. Cars frequently drove that way as well.

Meanwhile a small revolution was underway in electronics. Really small, actually: the microchip. This new-found computing capability made fuel injection possible at a reasonable price, and the improvements in fuel economy and drivability, plus the relative ease of meeting emissions requirements with the microchip quickly put the carburetor on the endangered species list.

The last production car with a carburetor sold in the US was the 1991 Ford Crown Victoria Police Interceptor equipped with the 351 cubic inch (5.8-liter) V-8 engine. Not surprisingly, komrade, the Russian Lada was the last production car with a carburetor. NASCAR race cars still use carburetors, but that’s an artifact of that racing league’s history rather than a performance-enhancing factor.

The new fuel-injected engines had their injectors in the intake tract just outside the intake valve and were a huge leap forward from carbureted engines. However, they had a major shortcoming. The air-fuel mixture coming into the combustion chamber was “homogenous.” It had the same air-fuel ratio for the whole lump of intake mixture that was drawn into the cylinder.

Engineers had long known, however, that varying the ratio in the intake charge so that the mixture was richer (a higher ratio of fuel to air) around the spark plug and leaner near the cylinder walls would be more efficient and cleaner at light loads. In the seventies, this “stratified charge” had been a goal of Ford engineers who worked with shaping the flow of carbureted air coming into the intake chamber—it never worked to satisfaction—and with spraying fuel directly into the combustion chamber. (This had been done before with the 1954 Mercedes-Benz SL300 among others). That worked but it was too expensive and the supporting technology just wasn’t ready yet.

Flash forward to the late nineties and later. Direct injection—manufacturers have their own pet names and abbreviations—has become increasingly popular among carmakers and for good reason. Fuel economy is improved and more horsepower can be extracted from an engine. In part that’s because the stratified charge effect that direct injection makes possible, and also because the evaporative cooling from the vaporization of fuel in the cylinder allows a higher compression ratio which increase power and fuel economy.

For example, the 2009 Cadillac CTS with the standard 3.6-liter V-6 with conventional fuel injection was rated at 263 horsepower while its direct-injection counterpart make 304 horses, doing this with essentially the same fuel economy 18/26 mpg city/highway for the standard engine and 17/26 mpg for the more powerful direct injection model. That was the final year for conventional port injection to be used in a Cadillac CTS.

Direct injection will be used more widely in the future. Another enhancement will be “piezo injection”, which relies on the ability of quartz crystals to change shape when exposed to electric current. Used in fuel injection, this allows very finely calibrated spritz and is an advancement over “solenoid-metered injection.” Instead of the single pulse per combustion event of the solenoid injectors, piezo injectors can pulse as many as seven times, giving engineers even more charge shaping to play with.

Ford Motor Company introduced direct injection with turbocharging in the 2009 Lincoln MKT crossover. The 3.5-liter engine delivered 340 horsepower and 340 lb.-ft. of torque. Dubbed EcoBoost, direct injection with turbocharging will be taking over the vast majority of Ford gasoline engines in the near future, including a 1.0-liter three cylinder–the smallest engine Ford has ever made–that will perform more like a 1.5-liter four.

Indeed, it’s only a matter of time before gasoline-fueled engines with conventional port fuel injection are viewed as primitive as engines with carburetors. At least now, unlike some athletes who didn’t know—really!—what was in those hypodermics, you know what direct injection means and why it’s good for you.

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First published Examiner.com

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Multiple award-winning author John Matras is founder and buzzard-in-chief at CarBuzzard. He has written about and photographing cars, including books, dead tree and on-line since 1980, fired by the best and some of the rest. John primarily runs and writes for CarBuzzard, also freelancing elsewhere. More about John