PeachParts Mercedes-Benz Forum - View Single Post - Why do some diesels have glowplugs and some not?

katja · #45 02-04-2007, 01:07 AM

My understanding is that there are two main types of diesel engines: Indirect and direct injection. Directly injected diesels have almost always been used on large trucks, buses, farm equipment, boats, generators, etc. Fuel injection for diesels until recently was all-mechanical so they were smoky and noisy especially when cold. So, when Mercedes put a diesel engine into a car, they developed the prechamber concept (indirect injection) to help with some of the noise and emissions. At least this is what I remember reading in a book about Mercedes-Benz when I was on vacation a few years ago. I believe all the other automobile & light truck engines - VW, Peugeot, Izuzu, Ford, GMC etc used the prechamber indirect-injection design.

So, in a direct injection diesel the heat of compression is enough to easily ignite the fuel when injected (at least in temperatures above freezing), but in an indirectly injected diesel the fuel is injected into the prechamber first, and the prechamber takes a much longer time to heat up from compression alone...so you need to heat the air in the prechamber with a glow plug or some other type of heater. IIRC someone on this forum described the prechamber as kind of a "heat sink" where it draws away some of the heat from compression....I guess that's why the OM6xx engines don't start at all when cold if no glow plugs work.

Interestingly, I remember seeing an ancient stationary diesel engine in a museum in England and the display caption described using a glowing hot "punk" type stick, that the operator had to put into the pre chamber to heat the engine before starting.

Now the common-rail, electronically fuel-injected direct-injection diesels such as the CDI, are quieter than even indirect injected diesels with their new technology. but they still have glow plugs or intake heaters to make starting easier in very cold temperatures, but don't really need them above freezing. Older DI designs may have had more crude cold start aids for below-freezing like an ether injection system.