Why do diesels need such complicated injection systems such as HEUI or high pressure fuel pumps and gasoline engines dont? I'm guessing its because the fuel going into the cylinder has to be more precise?

 

I am not an expert. I just am making some educated conclusions based on what little diesel theory that I know.

First, the compression in the cylinder on a diesel engine is very high. Fuel is being injected at or near TDC, when compression is highest. Because of this, you need more pressure to push the fuel into the cylinder.

"Fuel injection" in gas engines is really a misnomer. The fuel isn't injected, it's sprayed out of a nozzle into the airflow above the intake valves. It doesn't take much pressure to do that. (Note: There are new Direct Injected gas engines that do inject fuel directly into the cylinder. I'm not referring to those).

Second, you want to get liquid fuel to come out as a vapor as much as possible. Liquids don't burn, vapors do. Pushing fuel through a very small hole at very high pressure will tend to vaporize it much faster than pushing fuel through a larger hole at a lower pressure.

Gas engines want to vaporize the fuel also, and there fuel injectors are intended to do that as much as possible, but gas engines also have time for the fuel to travel through the intake, valves and into the pistons, during which the fuel can vaporize.

Those are just some reasons that I can think of.

 

Basic info:

HowStuffWorks "Diesel Fuel Injection"

 

So I guess its mainly because of the direct injection. Makes sense now that I think about it. Thanks!

 

There are many reasons diesels have a highly complicated injection system. I'll briefly explain the main ones.

1) Fuel: diesel and gasoline are different. Diesel fuel is heavier, and evaporates far slower than gasoline. Because of the properties of diesel vs gasoline, diesel fuel requires higher injection pressures to atomize the fuel into small enough droplets to burn efficiently. Gasoline simply burns quicker and atomizes easier. Diesel burns slower and longer. In order to speed up that burn, the fuel needs to be sprayed out in smaller droplet sizes. Achieving this requires much higher injection pressures on a diesel engine than gasoline engines.

2) Ignition source: diesel engines don't use spark plugs. We all know that. It relies on compression and heat to ignite the fuel. Since there is no initial spark to start the combustion process, the fuel ignites easier the more it's atomized.

Now combine those two principles to see where diesel injection is heading. Years ago, diesel fuel used to be injected into pre-combustion chambers. These engines were all known as "indirect injection". These engines still injected fuel at higher pressures than gasoline engines, but far far lower than what we see out of today's diesels. Diesels now can see injection pressures of well over 20,000 psi with direct injection. So what's the point?

Technology has advanced to give us the ability to inject diesel fuel at higher pressures and with more precision. What this has done has given us the ability to vastly increase the efficiency of a diesel engine. Now we can eliminate the pre-combustion chamber, and inject the fuel directly into the cylinder. This helps to increase thermal efficiency by reducing the amount of heat loss from the pre-combustion chamber. But in order to make it work, the fuel has to be injected at extremely high pressures. Removing the pre-combustion chamber requires the fuel and air to mix far quicker, so the fuel must be in an extremely fine mist in order to be able to ignite without a spark.

That's why diesel injection systems are so complicated. It takes quite an effort to effectively atomize the heavier fuel and inject it in a very precise amount in an extremely small window of time in order to achieve a very efficient and thorough burn. Gasoline engines don't have those issues. The fuel they use is lighter, atomizes far quicker and easier, and flashes quickly. On top of that, gasoline is not injected directly into the combustion chamber, so the injection timing is not what's keeping the engine running (it's the spark timing instead).

Hopefully that helps explain some of the major reasons. The technology has reached the point now where higher injection pressures on diesels allow these engines to create massive amounts of power at extremely high RPM's.... I'm talking diesel motors reaching 6,000+ RPM's. It has also helped to make these engines more efficient, as well as blow out less smoke (even without current emissions equipment). Of course, we can see how diesel engines have advanced in terms of power, as the past 2 decades have seen these motors become real responsive, peppy, and strong.... compared to older diesels that were sluggish, smokey, and slow.