# Fuel injection mechanism

It is hard to imagine injecting spray of fuels at several thousands rpm. How is it possible to pump liquid fuel at such high frequency?

On a four cycle (four stroke) engine with sequential port fuel injection, the fuel injectors fire at 1/2 the rate of the RPM (ie: 3000 rpm engine speed equates to 1500 injector pulses per minute), which just happens to be the same speed at which the spark plug for a single cylinder fires. Four factors allow the injector to operate at this speed.

The first is, electricity moves at the speed of light. This gives the injector an advantage over the speed of the engine. When designing the injector, the engineer needs to think about the harmonics of the injector. This means the injector must be able to close fast enough to completely shut off the flow of fuel at the maximum speed at which it is meant to operate. This is done by putting in the right size and weight of spring. To open it, an electromagnet is used. This can move a hunk of metal, while not at the speed of light, very fast. The Engine Control Unit (ECU) will tell the injector to stay open for a period of time (called the pulse width), which in conjunction with the next two factors, allows the proper amount of fuel to enter with consideration to the operating needs of the engine (speed, load, etc.).

The second part of this equation is the orifice size of the injector. This allows a certain amount of fuel to be injected at any given time.

The third part is the pressure at which the fuel is introduced into the system. If all else stays the same (pulse width, injector size, etc.), as you increase the fuel pressure, the amount of fuel which passes through the injector increases. For instance (and these are arbitrary numbers), if a fuel system is under 30psi pressure and an injector is rated at 30 pounds per hour rate, if you increase the pressure to 45psi (1.5 times the normal pressure), you could expect to see a rate of 45 pounds per hour come out of the injector. There is a point at which this will become moot, as a given orifice can only flow so much fuel, but that is another story.

The fourth part of our tale is this: it really doesn't take much fuel to run an individual cylinder of an engine. If a small four cylinder engine gets 40mpg, how many actual revolutions of the engine does it take to get it down the road? Let's break down the math. Let's assume our small make believe car running down the road at 60mph does so with an engine speed of 2000 rpm. The 60mph equates to one mile every minute. So every mile you travel, your injector is only firing 1000 times, or almost 17 times per second. This really isn't all that fast, when you consider most motion pictures run at 24 frames per second and the eye can see at a frequency of ~60Hz. What does all this mean? The injector does not need to be open for very long nor does it even need to be opened very far to allow enough fuel to get into the cylinder to allow it to operate.

• Good point, @Zaid. I had never seen a measurement about the ratio when considering volume, but it definitely brings home the point. Thanks. May 24, 2014 at 19:19
• Deleted my previous comment since the stoichiometric ratio by volume is 11,500:1
– Zaid
May 25, 2014 at 7:10