What Does MPI Mean on a Car?

by Robert Allen
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One of the most important components of a well-functioning engine is the fuel system. Modern cars tend to be fuel injected, rather than using carburetors, but not all fuel injection system are created equal. Multi-point injectors are more complicated than single-point systems, but they lead to an engine with better fuel economy and cleaner emissions.

Getting Fuel to Your Engine

In general, pure fuel combusts poorly --- for peak performance, the engine must mix the fuel with a precisely metered quantity of air. Early engines used a mechanical device known as a venturi. Air flowing over the venturi draws fuel into the airstream, delivering a fuel-air mixture to the cylinders. These devices, known as carburetors, suffer from inherent inefficiencies, leading to uncombusted fuel and high emissions. As a result, fuel injectors emerged --- small devices that inject a predetermined amount of fuel, rather than relying on air pressure to do the job.

Single vs. Multi-point

Some fuel injectors, like carburetors, mix in fuel at a single point in the throttle body. This makes it easier to upgrade engine designs for use with injection rather than carburetion, but there is a lack of improved fuel efficiency. Multi-point injection (MPI) systems inject fuel at each cylinder, allowing much greater control over how much fuel the engine burns. In MPI systems, the injection generally takes place directly inside the cylinder's intake valve.

Types of MPI Systems

Depending on how the injection is handled, MPI systems fall into several types. Simultaneous MPI engines inject fuel into all the cylinders at the exact same time; this is similar to single-point injection system, in that the fuel delivered to any given cylinder at any given time is the same in both cases. Batch MPI systems inject fuel into groups of cylinders. In a piston engine, cylinders are at different points of their combustion cycle at different times, so a batch system can be used to inject fuel to cylinders in a similar part of the cycle. Sequential MPI systems are more complicated, and time the injection of fuel so that it enters each cylinder exactly when it's needed.

Advantages of MPI

Compared to single-point injection or carburetion, MPI engines have better fuel economy and lower emissions. This is because the engine can meter the amount of fuel needed for each cylinder. If too much fuel is added, there are leftover hydrocarbons, and carbon monoxide is formed from the combustion; if too little fuel is added, the mixture can explode early, leading to engine "knock." Multi-point injection, in combination with other technologies, can also improve fuel efficiency by allowing the engine to selectively disable certain cylinders during cruising operations when the engine's full power isn't needed.

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