This question has two correct answers, because people ask it in two different ways. Some want to know how many injectors physically sit in the engine. Others want to know how many times an injector opens during one cycle. The first answer is usually the number of cylinders. The second can be a surprise, because a modern diesel fires several times per stroke.
First meaning: how many injectors the engine has
The rule is simple and holds in most cars without exception: one injector per cylinder. A three-cylinder engine has three, a four-cylinder four, a six-cylinder six. It does not depend on displacement but on cylinder count, because each cylinder needs its own dose at its own moment.
That is worth stating plainly, because a belief circulates that the number of injectors follows engine capacity. A one litre three-cylinder has three injectors and a one and a half litre four-cylinder has four, despite the small gap in capacity. The cylinder layout decides, not the litres.
The rule applies equally to port injection, where the injector sits in the intake manifold, and to direct injection, where it enters the head. We covered the differences between those designs in the article on types of injectors.
Three exceptions worth knowing
Single-point injection. Cars from the turn of the eighties and nineties used a layout in which the whole engine had one injector, mounted where the carburettor used to sit, above the throttle. Fuel entered the shared manifold and travelled to the cylinders with the air. A simple arrangement, found only in older cars today.
An LPG system. A car running on gas carries two sets at once: the original petrol injectors and separate gas injectors plumbed into the intake manifold. On a four-cylinder that makes eight. Worth remembering, because idle petrol injectors can gum up over months of disuse.
Dual injection. Some modern petrol engines have port and direct injection simultaneously, meaning two injectors per cylinder. The ECU switches between them according to load, and fuel from the port injector washes the intake valves along the way, limiting carbon build-up there. Eight injectors in a four-cylinder engine therefore need not mean eight cylinders.
Second meaning: how many injections per cycle
Here the answer is more interesting. In a nineties engine the injector opened once per cycle and delivered the whole dose at once. In today’s common rail diesel one cycle usually means several separate injections following each other within fractions of a second.
| injection | when | what for |
|---|---|---|
| pilot | just before the main one | softens the pressure rise, quietens combustion |
| pre-injection | right after the pilot | prepares conditions for ignition |
| main | around top dead centre | does the work, moves the car |
| post | shortly after the main one | reduces the soot being formed |
| regeneration dose | late, during the exhaust phase | raises exhaust temperature for the filter |
Not all of them occur together. The ECU picks a set according to load, revs and temperature, so gentle driving may involve three, a cold start more, and the latest piezo systems split the dose more finely still. The key point is that all of it happens within a single crankshaft revolution, at hundreds of revolutions per minute.
Why split one injection into several
Mainly so that a diesel stops sounding like a diesel. When the whole dose ignites at once, cylinder pressure rises abruptly and you hear it as that characteristic knock. A small pilot dose delivered earlier ignites first and warms the chamber, so the main dose burns more gently. The engine quietens down, and nitrogen oxide emissions fall along the way.
The post injection works the other way round in time: it comes after the main one and helps burn off soot that has already formed.
The dose delivered late, during the exhaust phase, has a separate role. It does not move the car at all, it heats the exhaust to the temperature the particulate filter needs. That dose is responsible for higher consumption during regeneration and for diluting the oil, which we write about in the article on DPF and fuel consumption.
What this means in practice
The more injections per cycle, the smaller each individual dose and the greater the precision demanded of the injector. And here is something worth knowing.
The smallest dose fails first. A worn injector will still deliver the main dose reasonably well, because it is large. But the pilot dose, measured in single cubic millimetres, it can no longer meter accurately. You can hear the result: the engine starts knocking at idle, especially when cold, even though it still drives perfectly normally.
So noisier idling in an older diesel is often an early sign of injector wear, long before smoke or power loss appear. How to confirm it by measurement is covered in the article on injector reconditioning, and cleaning methods and their limits in the article on what to clean injectors with.
A correction: carburettors and injectors are mutually exclusive
This needs setting straight, because the phrase “injectors mounted in the carburettor” appears online surprisingly often and contradicts itself.
A carburettor and fuel injection are two different, mutually exclusive ways of delivering fuel. A carburettor draws it in by vacuum, using a narrowing in the tract, and needs no injector at all. Injection delivers fuel under pressure through an injector and needs no carburettor. A car has one or the other, never both.
The confusion arises because in single-point injection the injector sits roughly where the carburettor used to, in the throttle body. That is where the association comes from, but it is still injection, not a carburettor with an injector in it.
How to work out what your car has
The simplest route is to count the cylinders and assume one each. The registration document or a parts catalogue gives the cylinder count.
Then look under the cover. Injectors plumbed into the intake manifold mean port injection. Injectors entering the head, with a thick metal pipe running from a shared rail, mean common rail. If you see an extra rail in the manifold with its own fittings and hoses, that is an LPG system and you have two sets.
The number of injections per cycle cannot be counted by eye. It is an ECU parameter, visible in a diagnostic tool as separate doses, and it changes on the fly depending on how you happen to be driving.
