German engineering advances of the past ten-years have included numerous engine and power-train modifications. This issue is focused on the pros and cons of direct injection engine design. Direct injection is the method of how fuel is delivered into the engines combustion chamber.

Direct Injection:

Direct injection is used to deliver fuel directly into the engine’s combustion chambers as opposed to traditional fuel injection that sends fuel into the intake manifold or plenum allowing it to wash into the combustion chamber. This is accomplished by moving the position of the fuel injector so that it is located directly in the combustion chamber similar to the spark plugs.

With the advent of piezo injectors, engine developers can now inject fuel into a cylinder’s combustion chamber in just 0.2 milliseconds. This allows the engine management system to make multiple fuel injections during a single combustion cycle improving fuel efficiency and controlling engine performance real time. One example of this technology is BMW’s N53, N54 and its successor, the N55 engines another is the popular Volkswagen/Audi TDI diesel engine producing close to 50 mpg.


Although direct injection design has many benefits, there have been significant drawbacks related to the direct injection process. Most notable as the camshaft timing overlaps, there is a tendency to pull carbon deposits from the combustion process into the engine ports.  In traditional engine design, fuel entering the engine through the intake manifold will have a tendency to wash deposits away.  With the addition of detergents that are added to gasoline, this helps keep the engine valves clean.  With direct injection engines, these detergents are useless as the fuel enters in the combustion chamber itself. Clogging of the intake and exhaust ports can lead to poor performance due to reduced air and poor valve seating.  Some engine designers are now experimenting with adding a traditional style injector for the sole purpose of cleaning the ports.

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