Fuel injection: port vs direct injection

Every petrol engine with fuel injection sprays its fuel through injectors, but there are two places to put them, and the choice changes a lot about how the engine works.

• Port injection puts one injector in each intake port, the passage in the cylinder head that leads to the intake valve. It sprays onto the back of the valve, and the fuel goes into the cylinder with the air. The in-tank pump alone feeds it, at about 4 bar (58 psi). How fuel gets into the engine shows this system.
• Direct injection puts the injector in the combustion chamber itself, next to the spark plug or in the side of the chamber, and sprays the fuel straight into the cylinder. It needs far higher pressure, typically 200 bar (2,901 psi) and up to 350 on newer engines, from a second pump driven by the engine.

From the 1990s to the 2000s, almost every petrol car had port injection. Today most new petrol engines use direct injection, often shortened to DI or GDI, for gasoline direct injection. Makers have their own names for it, such as TSI, FSI, EcoBoost, Skyactiv-G and D-4. Diesels have always injected directly; that’s how they light their fuel.

The demo runs one cylinder of a 2.0-litre non-turbo engine flat out at 6,000 rpm. The buttons choose how it gets its fuel, and the slider adds miles. Use Next stroke to step through the cycle.

• Start on Port. The injector sprays onto the back of the intake valve during the exhaust stroke, while the valve is still shut. The fuel sits on the hot valve, starts to evaporate, and rushes in with the air when the valve opens. At high revs there isn’t time to spray it all onto a shut valve, so the injector keeps going as it opens.
• Press Direct. Now the injector stays quiet until the intake stroke, then sprays straight into the cylinder. The charge temperature drops from 35°C (95°F) to 20°C (68°F), because the fuel takes its heat from the air as it evaporates. Port fuel mostly takes its heat from the valve and the port walls instead.
• The cooler charge is denser, so more air fits in, and it resists knock, so this engine runs a 12:1 compression ratio instead of 10.5:1 on the same fuel. Together they give about 9% more power, and the higher compression also means better economy.
• Slide the miles up. With direct injection, nothing washes the back of the intake valve, and carbon builds up on it. By 60,000 miles it blocks 6% of the air, and the engine idles roughly.
• Press Port and direct. Many newer engines have both. They spray mostly direct when working hard, and use the port injectors at light loads and from cold, which keeps the valves clean.

Why so much pressure? A port injector has the whole cycle, two turns of the crank, to spray in gently at 4 bar (58 psi). A direct injector has part of one stroke, about 5 thousandths of a second at 6,000 rpm, and it’s spraying into the cylinder, where the pressure is rising as the piston comes up. It also has to break the fuel into droplets fine enough to evaporate before the spark. Hundreds of bar does all three. A direct injector can even spray several short bursts in each cycle, which helps the engine start cleanly from cold and heat up its catalytic converter quickly.

The model is simplified. Real engines differ a lot in how much they gain from direct injection, and in how quickly their valves carbon up.

The idea is old. The Mercedes-Benz 300 SL of 1954 had mechanical direct injection, and it was common in aircraft engines in the Second World War. But making it work well, cheaply and cleanly in an everyday car needed modern electronics.

Mitsubishi brought the first mass-produced direct-injection car engine to Japan in 1996, and VW, Audi, Toyota and others soon followed. These early engines tried an extra trick at light loads, called stratified charge: a small, rich cloud of fuel round the spark plug in a cylinder full of spare air, which used very little fuel. It made a lot of nitrogen oxides that were costly to clean up, so most makers dropped it.

Modern direct injection fills the whole cylinder evenly, at the right ratio for the catalytic converter, as Air-fuel ratios explains. What made it take over is the turbo. Small turbo engines need to squeeze hard without knocking, and direct injection’s cooler charge is one of the main things that lets them do it on everyday fuel.

Port injection

• Low pressure, from the in-tank pump: simple, cheap and quiet.
• Fuel washes over the intake valves, keeping them clean.
• Fuel has plenty of time to mix with the air, so it makes very little soot.
• When the engine is cold, some fuel sticks to the walls of the port, so the ECU has to add extra.

Direct injection

• Cooler, denser charge, so more compression or boost before the engine knocks.
• Better economy and more power from the same size engine.
• Fuel goes exactly where and when the ECU wants, so the engine responds quickly.
• A high-pressure pump and injectors that cost more and tick loudly.
• Carbon on the intake valves, and more soot, especially from cold.

• Carbon on the intake valves. The engine’s crankcase breather sends oily vapour into the intake, and many engines also feed some exhaust gas back in to cut emissions. On a direct-injection engine, that sticks to the hot backs of the intake valves and bakes into a hard crust. Over tens of thousands of miles it can cause a rough idle, misfires from cold and a loss of power. Fuel additives can’t help, because the fuel never touches the valves. The cure is to clean them, usually by blasting them with crushed walnut shells with the intake manifold off. How much carbon builds up depends a lot on the engine and how it’s driven: lots of short trips make it worse.
• Ticking. Direct injectors and the high-pressure pump make a fast, sewing-machine tick, loudest at idle. That’s normal.
• A failing high-pressure pump. The pump is driven by a lobe on a camshaft. If it wears, the fuel pressure falls, the ECU stores a fault code such as P0087, for low fuel pressure, and the car may lose power or go into limp mode.
• A leaking or blocked injector. One injector dribbling or spraying badly makes its cylinder misfire, and the ECU names the cylinder in a fault code. A direct injector that leaks into the cylinder can also wash oil off the bore.
• Fuel in the oil. Some direct-injected fuel lands on the cylinder walls, especially from cold, and slips past the piston rings into the oil. Lots of short trips can thin the oil, which is one reason makers specify the oil they do.
• Soot. Direct injection leaves less time for the fuel to mix, so it makes more tiny soot particles, especially from cold. Since about 2018, most new direct-injection cars sold in Europe have had a petrol particulate filter, like a diesel’s, to catch them.
• Low-speed pre-ignition. Small turbo direct-injection engines can suffer this at low revs and heavy load. Octane: what it actually means explains it.

More power means more air, and more air needs more fuel, so the fuel system is one of the first limits a tuner meets.

• Port injectors can be swapped for bigger ones, or the fuel pressure raised, to flow more fuel. The ECU has to be told about any change, or the mixture will be wrong.
• Direct injection is harder to upgrade. Often the high-pressure pump runs out first, and upgraded pumps with a bigger plunger are a common tuning part for popular turbo engines. Upgraded direct injectors are available for some engines, but cost more.
• Adding port injection. For big power or for E85, which needs 30% to 40% more fuel than petrol, tuners sometimes add a set of port injectors to a direct-injection engine. It also helps keep the valves clean.
• Catch cans fit in the crankcase breather hose and trap some of the oil mist before it reaches the intake, which can slow carbon build-up. They need emptying regularly, and some designs let unfiltered vapour out, which isn’t legal on a road car.