How Does Direct Injection Work?

Direct injection. Just about every car has it now, and those that don’t probably will in the next few years. It can add power, reduce emissions, and is a big part of why just about everybody is offering tiny turbo engines that make big horsepower. But how does it work? And why does my dealer want to blast walnut shells into my engine?

Electronic fuel injection works by using solenoid-controlled nozzles that precisely meter how much fuel goes into the intake of the engine. But while they can be very precise in how much fuel comes out, they aren’t so great at deciding where it goes.

That’s because they spray the fuel into the intake manifold air stream. Once in that air stream, it can go just about anywhere. Into the cylinder you want, into the cylinder you don’t, or it can even form a puddle in the manifold. That can be catastrophic, though it’s definitely not common. But that was the best automakers could do. The fuel needed that airflow and the space of the manifold to mix with the air. Fuel that’s mixed into as a vapor combusts. Liquid fuel doesn’t.

Ideally, you want the fuel to be injected as close to the actual cylinder as possible. But the heat of combustion and packaging constraints meant that somewhere in the intake manifold was the best automakers could do. In a gas engine, at least.

That’s because diesel engines used something different. Diesels don’t have spark plugs. And they need very well mixed and atomized fuel because the only things making the fire happen are heat and compression. Which is really just more heat. So for decades, big diesel engines have had fuel injectors where the spark plug used to go. Fuel was delivered at up to 36,000 psi. Compare that to a carburetted car at less than 10 psi and a conventional gasoline fuel injected vehicle at around 70 psi.

Those high-pressure injectors were massive. And that much fuel pressure required expensive pumps. Usually more than one of them.

Automakers and suppliers, like Ford and Bosch, experimented with direct injection of gasoline but reliability and cost concerns meant that neither project went far.

Then in 1996, direct injection gasoline hit the automobile market. With Mitsubishi, surprisingly. The Galant’s 1.8L four pioneered the tech in cars.

What direct injection does is put a fuel injector directly into the cylinder. Usually through the cylinder head, right beside the spark plug. Though it can come in from the side of the combustion chamber as well. That means the business end of the injector needs to be strong, heat resistant, and very small. Oh, and it needs to deliver the exact right amount of fuel in a miserable environment.

Because it’s spraying directly into the cylinder, it needs to spray at very high pressure. That tens of thousands of psi number like a diesel. Otherwise, it would just leave a puddle of fuel on the piston. And that’s bad.

Spraying the fuel directly into the chamber means that it’s much more precisely metered. Not just the amount, but when in the combustion cycle it’s injected. That means better air-fuel ratios. Both for power and for economy. The fuel is so finely atomized and it’s there at just the right time so that it doesn’t actually have to take time to mix in the combustion chamber. It stays as a vapor so compression ratios can be higher. More compression means more power and more efficiency too.

As an added bonus, when the fuel is atomized into the combustion chamber the evaporation of the fuel actually cools the combustion charge. Changing from liquid to vapor removes heat from the surrounding area. That cooler charge reduces knock and allows for more compression (or more boost) and more ignition timing. Again, more power when you’re on it and better fuel efficiency when you’re not.

That precision also allows for better and more thorough use of variable valve timing and variable lift. Letting those technologies do things with the valves that wouldn’t otherwise be possible.

That direct injection and better turbos are why tiny displacement engines now make the power that just 10 years ago would take a V6 double the size and significantly more fuel-hungry. Bosch says that DI offers CO2 emission and fuel consumption reductions of 15 percent. That’s a big deal.

It’s not all sunshine and rainbows, though. The transition to DI wasn’t as simple as new parts and a new hole in the cylinder head. Those new parts, like the extra high-pressure fuel pumps and stronger injectors, are more expensive. So are turbos, though DI engines that are naturally aspirated are still more powerful and efficient than a non-DI equivalent.

A faulty injector can lead to fuel pooling on the piston. That fuel can remove the oil film from the cylinder wall and lead to corrosion or increased wear. It can even run down into the oil pan and dilute the oil with gasoline. Not good for bearings.

So what about those walnut shells? Well, direct injection can also lead to a buildup of carbon in the intake. Especially on the back of the intake valves. That’s because no matter how good the valve seal is, a small amount of combustion can leak through. That leak leaves carbon on the valves. Basically soot. If too much soot builds up, it can stop the valves from sealing properly. Then even more combustion leaks out. Or if it builds up badly enough, chunks of carbon can fall into the cylinder.

That external combustion happens in both DI and non-DI engines, but there’s an important difference. Gasoline is a solvent. That means it dissolves things. Like carbon. The fuel sprayed on the back of the valves by port fuel injection cleaned the carbon off. DI engines breathe only air so there’s no fuel on the back of the valve to clean it. It’s not a problem that will happen to every car or driver, but it can happen. The fix? Clean out the carbon.

That’s done by removing the intake manifold and attaching a device that sprays crushed walnut shells (or a similar abrasive) down the cylinder head and onto the back of the valves. Pro-grade tools run a nozzle inside a vacuum attachment that sprays the shells and sucks them back out of the head, keeping them out of the engine.

There’s another fix too. Automakers are starting to add a conventional port injector in addition to the direct injector. The port one is used in certain situations and keeps the back of the valves clean.