The pre-combustion chamber design seems like a dinosaur these days. Many had it and it is harder to start, when cold, than direct injection. I think the main reason they are harder to start, at the same compression ratio as direct injection engines, is because they have a much higher surface area to volume ratio in the combustion area. So at low cranking speeds the air has time to cool, or stays cooler, before injection. Even Mercedes diesel car engines from the '80s needed glow plugs with over 20 to 1 compression and pre-combustion chambers. It may have evolved from the hot bulb design, not as a hot surface, but as a way to better burn low grade oils by increasing the turbulence. It sort of fires a hot, rich, turbulent, burning mixture into the main combustion area where the burn is completed. Once started the whole sequence happens fast enough, even at idle, to keep going.

That's why the Cummins is so good at starting, even with a relatively low compression ratio. Direct injection into a large combustion chamber where the cool surface is far from the center of the hot compressed air. Far enough and calm enough for the air to be warmer at injection.

 

awsome videos! i kind of wish my auto shop teacher would have taught us about the older engines with the hot bulb. i think it would have helped a little more to get the idea across to some of the other kids. anyways great video!

 

Exactly. All that metal surrounding the heated air acts like a big heat sink. And this is also the reason a six cylinder diesel is easier to get started than a V-8 (assuming roughly equal displacement and compresion). More surface area = harder cold starts.

Why a PowerStroke needs glow plugs and a Cummins doesn't.

IDI engines will burn dang near anything oily. An IDI engine is also less sensitive to timing, so they run cleanly and efficiently over a wider RPM range. They are also quieter due to the "2 stage" nature of combustion, there is much less "diesel clatter". All this was much more important before there were electronic injection systems, now with infinitely variable timing and multiple injection events per combustion stroke these advantages have disappeared. And IDI engines are inherently less efficient - which comes back to the "surface area/volume" theme, as well as pumping losses from blowing hot air in and out of the prechamber through a tiny hole.