This question comes up often, and I recently found a couple of cool pictures that help explain it.
IDI Engines:The acronym "IDI" generally refers to "
In
Direct
Injection. In this configuration diesel fuel is injected
indirectly into the combustion chamber via a "pre-combustion chamber" inserted into the head:
The IDI diesel engine was standard VW passenger car fare until mid 1997 and may or may not be coupled with a turbo. Engines of this vintage without a turbo are often referred to as
NA or
normally
aspirated engines. IDI engines uses a pintle-style nozzle that sprays fuel in a mist and are threaded into the head like spark plugs. Due to the heat-sinking capacity of the surrounding head IDI engines generally require the use of glow plugs on all but warm days.
The fuel system for an IDI engine is almost always 100% mechanically controlled via a Bosch VE distributor-type pump, although Europe got a late version of the AAZ engine that had some vestiges of electronic timing control. Throttle control is via mechanical cable to the throttle pedal.
TDI Engines:The acronym "TDI" generally refers to "
Turbo
Direct
Injection. In this configuration diesel fuel is injected
directly into the combustion chamber formed by a depression machined into the crown of the piston:
The Direct Injection engine started appearing in European VW passenger cars in 1991, in some North American Passats in 1996, and were in general circulation world-wide by mid-1997. In all but rare cases the direct injection configuration is equipped with a turbo, hence the confusion some folks have thinking their turbo-equipped IDI motors is a TDI. The nozzle sprays fuel in 5 discrete jets and is held into the head with a top-mounted clamp. Glow plugs are not generally needed until the engine temperature drops near freezing.
The fuel system on a TDI generally uses a electronically-controlled Bosch pump with an Engine Control Unit, and the throttle is drive-by-wire using a potentiometer-equipped pedal.
A primarily home-built variant of the TDI pump known as "
m-TDI" uses the TDI pump body and nozzles but substitutes parts required for manual (ie cable) throttle control.
A more recent version of the TDI design, commonly referred to as "PD" or "pump duse", uses individual injector/pumps that are driven off the valve camshaft to pressurize the fuel to extremely high pressures and spray it out a nozzle with 7 holes. North America got PD engines from 2003-2006.
In 2009 VW introduced the so-called "Clean" TDI design... a common rail system with a completely different injection system yet again.
For a variety of reasons, including more precise fuel control and fewer thermal losses, TDI engines generally obtain better fuel economy than their turbo-equipped IDI cousins.
Topic: What's the main difference between "IDI" and "TDI" engines?? (Read 49774 times)
