Why do some diesels require glow plugs and others don't?

[Raspy]

I didn't read all the answers, but in MY mind the answer is fairly simple.

ASSUMEING both diesels are in good shape, with good compression, for cold starts,

Direct Injection = faster starts without glow plugs

Indirect direction = needs glow plugs...

I have plenty of both and this IS the way it always works out.

I HATE GLOW PLUGS!!

Years ago, even the small Kubota's were DI and they started instantly, with out glow plugs!!

ALL of my real tractors start WITH OUT having to glow any plugs, they are all DI!

THANK the EPA for glow plugs, that's why the mfg's went to them...I remember when it all got started down that path and I didn't like it any more then than I do now!

Did I mention that I HATE glow plugs??

SR

BTW, the "grid heater" in the Cummins, is there more to keep the cold smoke away than it is to get the DI Cummins started! I have one of those too...

Please do a bit of research before making such statements.

First, glow plugs have NOTHING to do with the EPA. Injection didn't "get started down that path" because of the EPA. Look at the 1950's Mercedes diesels for instance, or the early Perkins, etc, etc ,etc. Glow plugs help indirect injection diesels start. They are needed because indirect systems have more surface area per volume in the combustion chamber. The charge doesn't heat up as much and won't start as well. The earliest diesels, or "hot bulb" engines had a hot surface all the time they were running because they were very low compression. Indirect is a carry over from that early design and an attempt to get better combustion. Direct injection has a larger distance from the injector to the cold surface of the combustion chamber. That means a better chance of hotter air where needed for starting.

Cummins B series engines are direct injection and are inherently easier to start. The grid heater is an external air stream heater that helps them start under extremely cold conditions. It's normally not needed. I never use mine and it starts like a gas engine in any weather. The heater turns off shortly after startup and has little to do with smoke. Any smoke during warm up is do to condensation or cold cylinder walls, not the grid heater.

 

[Raspy]

Many modern direct injection engines have gone to glow plugs for one primary reason, that being customer satisfaction.

This trend probably started with light duty pickups, many of which found their way into suburban households where rough starts and white smoke emissions were simply not tolerable.

Industrial engines followed suit, as clean starts and no white smoke became marketing issues for generators and equipment housed near or with farm animals.

I worked in Diesel engine development for many years and still remember a field complaint from lt late 70s from a farmer from Minnesota who had a tractor that filled his barn with white smoke and made his cows sick.

We released a service kit with higher compression pistons for that product, although that resulted in higher black smoke and a resultant slight power decrease.

Huh? Glow plugs are primarily for customer satisfaction? How about for starting? indirect injection requires glow plugs or intake manifold heating to start in cold weather. An engine that won't start is useless. Glow plugs have been around FAR longer than the 70's. Try 1950's Mercedes, for instance. Perkins has has intake manifold combustion heaters far longer than that.

Black smoke comes from too much fuel, poor atomization or restricted air. Not higher compression. White smoke comes from condensation in cold engines or oil burning in cold engines.

 

[finn1]

In a DI engine, yes, for customer satisfaction, believe it or not.

I worked in combustion development for 35 years. With all other things being equal, i.e. Fuel injection pressure capability, etc., a higher compression ratio (smaller combustion bowl) will produce more black smoke.

There are ways to improve the situation, specifically higher injection pressures, turbocharging, intercoolers, etc, but the basic fact remains that higher compression ratios are smoke limited sooner than lower compression ratios.

It's all combustion science.

Yes, glow plugs and other starting aids have been around for a long time, far longer than when I started working on engine design and development in the seventies.

White smoke, by the way is caused by late timing, low compression ratio ( too much ignition delay, low cetane fuel, etc.)

 

[Creamer]

I" but the basic fact remains that higher compression ratios are smoke limited sooner than lower compression ratios."

Can you explain what "smoke limited" means? Also is sooner meant in a time or in terms of completeness in hydrocarbon combustion?