Why IDI diesels require glow plugs

[Carimbo]

A clear explanation of this subject is in this "Chevron Products Diesel Fuels Technical Review," section 6, Diesel Engines (choose) DIRECT-INJECTION AND INDIRECT-INJECTION

"The more rapid mixing of fuel and air achieved in IDI engines comes at a price, however. The high velocity flow of air through the narrow passage connecting the main cylinder to the prechamber, as well as the vigorous swirling motion in the prechamber itself, causes the air to lose significantly more heat during compression than it does in a DI engine. Coupled with a pressure drop from the main chamber to the prechamber, this results in an air temperature in the prechamber after compression that is lower than that in a similar DI engine.

Since rapid fuel autoignition requires a certain air temperature, an IDI engine needs a higher compression ratio to achieve the desired air temperature in the prechamber. IDI engines operate at compression ratios of about 20:1 to 24:1; while DI engines operate at ratios of about 15:1 to 18:1. The heat losses that necessitate these higher compression ratios have another, more important effect: they decrease the efficiency of the engine. IDI engines typically achieve fuel efficiencies that are 10% to 20% lower, on a relative basis, than comparable DI engines.

Even with the higher compression ratios, IDI engines may still be hard to start. Most IDI engines use glow plugs to heat the air in the prechamber in order to make starting easier. Glow plugs, which are small resistive heaters, are usually powered for only the first few minutes of engine operation."

Is this telling us the IDI engine suffers greater air temp. loss, therefore causing harder starting?

A nicely formatted (PDF) version that includes this section can be found Here.

[asavage]

Why IDI diesels require glow plugs

Lister CS diesels don't have glow plugs. Neither do most of the Indian-built Lister clones, though there are aftermarket GP kits available for some. Listers are IDI engines.

The original CS engines (through about 1958?) had a variable compression ratio system: higher compression for starting (and loads up to 1/3 capacity), lower compression for running at > 1/3 capacity. It worked via a hand-operated valve that connected an appendix-like port to the combustion chamber (or maybe the prechamber, I don't recall). Closed, the extra chamber volume is unused and the compression ratio is higher. Opened, and the total volume of the combustion/precombustion chamber is greater, lowering compression.



Lister raised the base compression of the engine in the late 50's and did away with the CS valve, and the engines continued to cold start OK. But they retained the -- separate -- compression release (hangs open one of the valves) so you can crank the engine up to speed, then drop the valve holder to initiate starting.

(for an entertaining read on a Lister clone generator project, I can recommend Rocketboy's project. Fun stuff.)

[asavage]

Discussion of Lister cold start diesels is not off-topic in this thread; it's related to the Q: why does a diesel need a GP (or other cold start aid)?

[philip]

Is there a vehicle application where IDI diesel has no glow plug(s)? Something in the past 50 yrs?

[asavage]

The thread is not limited to autmotive: it reads "IDI diesels". If it read "automotive IDI diesels", that would sufficiently narrow the scope. I do not believe the "automotive only" scope is implied merely by the forum title.

And anyway, Listers are just cool to talk about

[philip]

SNIP And anyway, Listers are just cool to talk about

Okay , why do Listers get away with no glow plug(s)? 'splane Lucy!'

[asavage]

I think it has to do with 280 lbs of rotating mass in the dual flywheels, in addition to the crank.

[quoting rocketboy]

To start

* Fuel on
* Cutoff lever on (stop lever)
* Both exhaust valve lifters in (compression release)
* Start cranking and crank as fast as you can.
* Flick one exhaust lever to run and bang it slowly starts. Then light the other lever and 'voila.
* booka-chica-booka-chica-booka-chica-booka-chica.....

But I guess that really doesn't explain much, because even with a good battery in an automotive diesel, you can crank and crank and only see white smoke.

I don't know. But I'm interested in the answer, should you run across it.

[philip]

I think it has to do with 280 lbs of rotating mass in the dual flywheels, in addition to the crank. -SNIP-

But I guess that really doesn't explain much, because even with a good battery in an automotive diesel, you can crank and crank and only see white smoke.

I don't know. But I'm interested in the answer, should you run across it.

Agreed. Rotating mass generates -no- heat in the combustion chamber. Lowering the compression would generate even less heat.

So ... what else is going on? Plumbers blow torch to the head or intake manifold? Shot of starting ether? Some ratio of gasoline in the diesel fuel?

[asavage]

So ... what else is going on? Plumbers blow torch to the head or intake manifold? Shot of starting ether? Some ratio of gasoline in the diesel fuel?

Nope, apparently (from all accounts I've read) they just start. Which makes me wonder who the target audience is for the aftermarket GP "kit".

[later]

Take that back, I guess some folks are "needing" a bit of help (ether) starting the Listeroid (Indian-mfgrd Lister clone w/o CS valve) in the cold.

http://utterpower.com/glow_plug.htm

[philip]

SNIP- Take that back, I guess some folks are "needing" a bit of help (ether) starting the Listeroid (Indian-mfgrd Lister clone w/o CS valve) in the cold.

http://utterpower.com/glow_plug.htm

"Mac McQuaid runs his Lister daily on home made fuel, due to cooler weather, Mac uses a wisp of ether and even a pony motor for cold weather starting. His name for the McGhee made plug is 'HOTSHOT', and here is what he has to say.. "

Al, I smelled an urban legend. "When it's too good to be true..."

Very resourceful how McGhee designed the glow plug to replace the auxillary chamber and valve.

[asavage]

The CS valve hasn't been shipped on a Lister(oid) in fifty years, AFAIK. It was only really used on the Singles, but the head is used on all engines so the casting has the threaded port. It's filled with a plug from the factory, AFAIK. McGhee's isn't the only one.
Eric Lyons (Rocky Mountain Power Source) does one too:

[goglio704]

You guys are getting a little deep for me, and I don't pretend to be that versed in diesel ignition, but one thing does occur to me. The opening from the prechamber to the cylinder would restrict the airflow and rate of pressure change in the prechamber during the compression stroke. Does rate of change have any bearing on this?

[asavage]

"Mac McQuaid runs his Lister daily on home made fuel, due to cooler weather, Mac uses a wisp of ether and even a pony motor for cold weather starting. His name for the McGhee made plug is 'HOTSHOT', and here is what he has to say.. "

Al, I smelled an urban legend. "When it's too good to be true..."

Home made fuel. 'Nuf said.

[philip]

SNIP The opening from the prechamber to the cylinder would restrict the airflow and rate of pressure change in the prechamber during the compression stroke. Does rate of change have any bearing on this?

The CHEVRON article at the beginning of this thread alludes to a loss of air temperature as the compressing air passes through the passage to the prechamber. Personal conjecture: At cranking speed, I believe pressure equalization time (main & pre-chambers) is nearly instantaneous. The passage is about 3/8th" diameter and about 3/4" long.

"Coupled with a pressure drop from the main chamber to the prechamber, this results in an air temperature in the prechamber after compression that is lower than that in a similar DI engine. "

[oak4000]

ok not know what engine the truck has, but my delivery truck at work which is a 95ish international 18' flatbed tilt, with an inline 6 turbo non intercooled, does not have glow plugs and even on the coldest mournings it takes only a couple turns of the crank and it fires right up, longest time it has sat was like a week and it started within 10 or so secs, about the same as my wife gas pathfinder