IDI vs. DI

Written by TsTDI

Diesel engines have different means of injection and injection pressures that make them widely different from engine to engine. You must understand that IDI engines are less prone to failure compared to DI engines and the reasons why. So whats IDI and DI? IDI stands for indirect injection in which fuel is delivered into a pre-chamber before it reaches the main combustion chamber. Combustion starts in this pre-chamber and eventually reaches the main combustion event. This is beneficial to those who burn WVO because it allows more time for atomization and therefore a better opportunity at a complete burn for a fuel that is slower burning then diesel. IDI engines are based on a fuel/air mixture based off of fast moving air, slow moving fuel in an engine. Thus, the injectors are not as complicated as a TDI’s and can run lower fuel pressures.

IDI’s are a very good engine of choice for burning WVO for the above reasons. Longer time for combustion is great advantage. Less fuel pressure means less stress on the injection pump and because the pressures are less, IDI injection pumps are not built to as tight tolerances as a TDI pump. IDI’s drawbacks are obviously an old technology found in an older car. If you’re a person of comfort or want to look good in a car, chances are an 86 VW or an 80’s model Mercedes isn’t the most attractive option. Having airbags and anti-lock brakes are also nice amenities to have.

DI stands for direct injection, which is also where the TDI derives its name from (turbo direct injection). DI engines are based on slow moving air and fast moving fuel. Direct injection means the fuel is sprayed directly in the main combustion chamber; there is no pre-chamber, unlike an IDI engine. Fuel pressures are much greater and design tolerances are stricter. Injection pumps are built to tight tolerances and standards in which there is very little leeway for error. The injectors of a TDI engine have two stages of injection in which two spring pressures must be met before the injector "pops" or injects fuel. The first injection is called the "pilot injection" in which a small quantity amount of fuel is injected to raise cylinder pressure. The increase of cylinder pressure allows for a more complete burn when the main combustion event occurs. Pilot injection occurs when the first spring pressure has been met of roughly 190-220 bar. The second injection is called "main injection" in which the second spring pressure of roughly 300 bar is met and the injector needle is lifted that much further to allow fuel to be injected. In a DI engine, the most sophisticated part of the engine is its fuel system and in the case of WVO, is what we are trying to risk the most.

http://auto.howstuffworks.com/diesel1.htm

This website shows the basic combustion event of a DI diesel engine. Understand how it works to get a better understanding of what I will be explaining further into this guide.

DI engines can achieve adequate burn without the use of a pre-chamber through extremely high fuel pressures. In comparison to an IDI engine which runs fuel pressures anywhere from 1400 to 1700 psi (threshold pressure to "pop" the injectors) a DI engine runs fuel pressures in excess of 23,000 psi. Remember the IDI vs. DI principle is entirely different. IDI principles are based on fast moving air and slow moving fuel. DI principles are based on slow moving air and fast moving fuel. It is clearly evident that 1700 psi vs. 23,000 psi of fuel pressure is a tremendous difference worth being noted.

Since we are using WVO as a fuel, we are risking the most important aspect of the TDI engine, the intricacies of the fuel system. Since the TDI engine is entirely based on the principles of the DI engine above, if this principle (slow moving air, fast moving fuel) is not met, the engine will not run at its designed capability. If the fuel is not being atomized properly by standards of which the DI engine calls for, incomplete combustion will occur. Understand these differences. Fast moving fuel means atomized properly for combustion and timed in the correct fashion for optimal combustion efficiency. If either of these is not met, problems will occur. The TDI engine gives us problems with both of these factors.

http://vegburner.co.uk/dieselengine.html

This website further explains the differences between IDI and DI and has pictures describing both. This website is a great reference on vegetable oil burning theory.