Why do some diesels have glowplugs and some not?
 

A comment on another thread raises this question in my mind. Why do our 617's have glowplugs but my CAT 3208 doesn't? The CAT smokes a lot more upon startup at all temperatures, but it seems to start at low temperatures when there would be no hope of starting the 617 without glowplugs. I know some diesels have intake heaters etc, but the CAT doesn't. There must be some design difference that permits it to start at low temperatures without glowplugs but I don't know what it is. Can someone enlighten me?

I got some great help from this forum when I installed a 3208 in a Ford 9000.

http://boatdiesel.com/Forums/index.cfm?Forum_ID=93&CFApp=5&RED=UN1170475349078

Glow plugs
 

Some engines have glow plugs because their compression ratio is to low to start the engine without them. Your Mercedes Benz 617 will typically have a compression ratio of 26:1 when new and this falls over time. My Dodge Ram high output Cummins has a compression ratio of 30:1. Diesel engines squeeze the air making it real hot and larger engines and those with compression ratios of 35 or 40:1 often do not need glow plugs! Also, your Mercedes Benz will be started and stopped several times in a typical day and industrial diesels will run at a steady speed for days at a time. When the air/fuel mixture is squeezed really hard, it will burst into flame. It really isn't good for high compression diesels to be used for stop and go driving. Diesel engines used to drive freezer compressors often have a compression ratio of 50:1 or even more!! These engines do not need glow plugs or intake heaters because once they are started, they won't be shut down until it's time for their maintenance schedule. They go 24/7 nonstop until service time and at a constant RPM! Does this help?:D

Cat 3208 n/a had a ratio of 16/1. If I remember correctly the design of the piston top was shaped in a way to promote easier cold starts.

http://www.yachtsurvey.com/Catdata.jpg

That's true! I also forgot, some of these engines had ether start systems! That's why it says "do not use starting fluids that contain ether"!

Quote:

Originally Posted by Knightrider966 (Post 1408438) Some engines have glow plugs because their compression ratio is to low to start the engine without them. Your Mercedes Benz 617 will typically have a compression ratio of 26:1 when new and this falls over time. My Dodge Ram high output Cummins has a compression ratio of 30:1. Diesel engines squeeze the air making it real hot and larger engines and those with compression ratios of 35 or 40:1 often do not need glow plugs! Also, your Mercedes Benz will be started and stopped several times in a typical day and industrial diesels will run at a steady speed for days at a time. When the air/fuel mixture is squeezed really hard, it will burst into flame. It really isn't good for high compression diesels to be used for stop and go driving. Diesel engines used to drive freezer compressors often have a compression ratio of 50:1 or even more!! These engines do not need glow plugs or intake heaters because once they are started, they won't be shut down until it's time for their maintenance schedule. They go 24/7 nonstop until service time and at a constant RPM! Does this help?:D

Issue 1: Last time I checked the compression ratio of a 617 is @ 21-to-1

Issue 2: Cummins HO engines have @ 17-to-1 compression ratio

Most diesel engines have compression ratios between 18-to-1 and 21-to-1.

The law of diminishing returns comes into effect with compression ratios above 24-to-1 or so. The physical structure of the engine becomes so bulky to survive the compression/combustion forces, the combustion chamber shape/size is compromised, peak cylinder pressures exceed whats mechanically practical, etc. at least this is what I have read.... Besides that, the size and power of the starter required to spin an engine like this would be impractical at best.

I have no idea why a Cat 3208 doesn't need any preheating but it is an interesting topic.

RT

All of the 2 cycle Detroits that I'm familiar with never had any glow system. But, they wouldn't start below 32F. without ether unless they were brand spanking new.

I believe the need for glow plugs is mandatory on an IDI diesel due to the very cold environment of the prechamber. A DI diesel has an even chance of starting without glow plugs, but, it's no certainty on any of them when it gets really cold. Ether is the mode of choice to get them going and it doesn't appear to have any negative consequences if used judiciously.

Some very old CAT engines have a 14:1 ratio. I'm told they were very difficult to start.

Compression ratio depends a lot on duty cycle and usage as well as what part of the country you are in. The latter has to do with exhaust emmissions. Diesel engines in Alaska in sub zero temps and continous service will have enormously high CR's. Special series Dodge trucks with a HO Diesel 7100 P/NB Cummins have a CR of 30:1. These are rare and if you were a Chrysler engineer, like me, you could be lucky enough to get one! CR's of 50:1 are not impossible, but these require a constant high RPM to perform the way they were intended. CR's of 40:1 and higher are severe duty industrial engines, not intended for consumer use. The size of the starter motor would scare you to death and the batteries required would barely fit in the back of my Dodge Ram.

Thats why I said "most" diesels. There are some super-high performance tractor pulling diesels that use low compression ratios. IIRC, this creates more "space" for the incoming air and they more than make up for the low compression ratio with huge boost via retardedly large turbos.

I don't understand the exact reason why lowering compression and increasing boost allows more fuel and therefore more power without breaking stuff but apparently it does.

RT

Because lowering your compression ratio and increasing your boost has the effect of a variable compression ratio going from 18:1 to 50:1 varying with engine speed. Diesel engines can take a lot more than you think. Industrial diesels, such as large tractor or power generation diesels, are built to run for periods so long between rebuilds, the comparison would be like going 2.5 million miles on your car! We often used 28% nickel content in the block for rigidity. Connecting rods were made to specs that you would have to see to believe. Higher CR's mean better fuel economy, real important when you can't just go to the filling station when the tank runs dry and your whole frozen Tundra town is depending on it's power! Having generator problems when it's 73 degrees below zero can be a real PITA! Besides, at those temps, you'd never get it started until spring!:D

Nobody was talking about the operating compression ratio. We are discussing static compression ratios.

What if you put an intake air heater on a diesel benz?

If you can find the space, it will help starting just like the glowplugs.

I have one from a 5.9 Cummins. I just need to figure out the best way to mount it.

I operate a DD 6V92 W/ turbo & blower and it will start at 40F. It is plugged in below that, or just kept running on the fast idle ( 1,100 rpm).
I have heard that the compressions on the DD with the turbo were lower than without the turbo.
I am wondering as to why ???