Question on Internal Combustion & EGT

[FormulaPro]

I have this burning question keeping me up at night...

I run two stroke snowmobile engines so I am very familiar with what can happen with a lean condition = burn down, or melted piston. My question is why are four stroke gas engines not as susceptible to lean burn down?...and how do diesles equate in this...more fuel = higher egt = burn down. Basically the opposite of two stroke...less fuel = lean = higher egts = more power.

Confused!

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2001 2500HD LT 4x4 SB Ext Cab D/A CoverMaster Cap
2000 HD Softail 1550 Kit, SE Heads, HSR42 Carb, 97Hp/103lbFt @ RW!
2002 Polaris XC800SP 1.25 Track Clicker Shock Sonic Blue X Graphics!

[Showgood1]

Totally different animals. A four stroke is getting oil wether it runs lean or rich. If a two stroke runs lean not only is it not getting enough gas it isn't getting enough oil either. Plus the fact that the piston is being fired upon twice as much anyway. A diesel is completely different. One thing a gas motor 2 or 4 stroke fires by a spark plug after compression. A diesels compression is 4 times that of a gas motor. When air is compressed that much it gets very hot so when fuel is injected it fires. You might say a diesel runs on hot air. Ever notice that a diesel gets louder when it is cold? Instead of richening up the mixture it leans it out to help warm the engine. Hears one that blows my mind ever hear of a 2 stroke diesel? Hope this helps.

[Logan]

The big difference is that petrol engines (with the exception of some really lean burn stuff) generally operate rich, while diesels always operate lean.

So as you lean out a petrol engine, you approach stoichiometric, which is pretty close to the hottest temp you get. As you richen a diesel, you approach it from the other side. I've melted pistons leaning out a petrol 4 stroke.

A two stroke petrol engine adds the lubrication issue, in that when you lean out a 2 stroke, you reduce the oil flow while also increasing operating temps. You just see it a lot sooner than a petrol 4 stroke.

[dmax]

Model airplane engines are 2 stroke diesels.

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1994 K1500 Blazer 350 Loaded

[arveetek]

The old Detroit 671 (238 hp) six cylinder diesels are two-strokes. They were very common years ago, and they sound like they are a high-revving motor, but they aren't. They reason they sound that way, is because there is a power (combustion) stroke every time the piston is at the top of the cylinder, so it is actually firing twice as much as a four stroke engine. The 671 in my Dad's truck has a huge blower on the side of the motor, which blows intake air in at the bottom of the stroke. These blowers are what a lot of race car drivers use on their modified engines.

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Casey Flint
"The RV Guy"
1981 C-20, regular cab, long bed,
With '82 6.2L N/A zooming diesel!
And '90 700R4 tranny, 4.10 gears.
Dual 3" side pipes, sounds awesome!
Fully customized!!!
(Coming soon: Schwitzer A4LD turbocharger!)

[Logan]

Oops, forgot the other two strokes.

The model aircraft style two strokes still have lube in the oil, but generally run ruch with the compression altered to suit the fuel, or a straight glowplug which still runs rich. They have no timing control except the variable compression engines, so use a different set of rules.

The two stroke diesels have a sump and oil system, and as far as the combustion process is concerned obey the same rules as a 4 stroke diesel.

[DmaxMaverick]

Model airplane engines are not diesels. There are kits for some engines to allow them to run on diesel, but still require oil mixtures and glow plugs. Most of my model engines are 4 strokes. I also have Wankel engines (3 strokes http://www.62-65-dieselpage.com/ubb/smile.gif ).

The engines, for lack of a better description, are 50/50 compression/spark. They do have glow plugs and require them to start. The glow plug is also required to maintain combustion after started. A bad glow plug may start the engine, but will not continue running after the electrical power is removed. Even hot engines die when a glow plug fails. These engines are marvels of design and performance. The power/weight/displacement factors are astronomical. A .5 cid 2 stroke model engine can produce around 2 HP, enough to propel a 6 pound airplane up to 100 MPH. They run on a mixture of (varies) methanol, 20% castor/synthetic oil, and 25% nitro. There are also spark plug kits for the same engines (expensive) that are said to make even more power. Using the spark plug kits also gives the ability to control combustion timing. The fuel/air mixture is controlled by venturi and jets. In other words, a carburetor.

In any case, leaning the engines will cause overheating and burn down, even when using a 20% or higher castor oil mixture. I've seen aluminum mufflers with holes burned right through them. I run the engines rich, sacrificing a little power for longevity. Running too rich causes the glow plug to lose too much heat between combustion, and the engine dies as soon as the electrical power is removed. The engines are about as expensive to overhaul as replace. Enough rambling....

With a diesel engine, it can not be too lean, or rich if the fuel system is operating normally. There is no control of intake air. The RPM's and power is controlled by the amount and timing of fuel injected. Less fuel means lower RPM/power. More fuel means higher RPM/power. I've never seen or heard of a diesel damaged by a lean situation. A bad injector that leaks between injection can cause detonation and a burned piston. A diesel with a clogged injector (lean) will just cause a lower power situation. With less fuel, the heat is lower. Diesels can also burn pistons with a poor injector spray pattern. Heat goes where the fuel goes.

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2001 GMC Sierra SLT 2500HD 4X4 Ext. SB -D/A- INDIGO BLUE - Delivered November 2000

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[This message has been edited by DmaxMaverick (edited 05-29-2001).]