Engine Loading ?????

[sbc438]

OK, I've been crewing on an IHRA Top Dragster foe a year now. I'm a degreeded Mechanical Engineer but I'm having trouble getting my head around one concept that keeps coming up!!!!

Our engine is a 528 ci Blown Alcohal Chrysler (appx 1950hp). The one thing that keeps poping up is engine loading. It seems everything revolves around how the engine is loaded. If it's not loaded enough we run rich and go slow. If we overload we run lean and start eating up the engine.

For som reason I can't get my head around the concept of needing load to burn the fuel????? We have a 44amp magnito that in my mind should be able to burn all the fuel provided if you stuck it in neutral and floored it. Through seeing it I know that won't happen!!!!

Is this concept rooted in Newtons 3rd Law "for every acton there is an equal and opposit reaction"? This is the only explanation I can come up with ?? i can rationalize that if you have no load (on the engine) you can't apply a greater foece than the avalible load i.e. you only burn the required fuel to make the needed power to react against that load.

Am I right??

 

[sbc438]

It seems possible that one contributing factor can be higher load won't let the engine accelerate as fast thus decreasing the mean RPM over the run in effect increcing the time avalible to burn the A/F mixture.

We do compensate for this with ignition timing by advancing the ignition to start combustion earlier and yes, as load goes up you tend to remove lead (advance) so you don't detonate.

On our engine we "DO NOT" change timing!!!! It's set at 32 degrees full advance and it stays there. One degree can be the differance between getting down the track or breaking the reing lands off every piston.

So, (thinking outloud) it is possible that we are cooling the charge as an equal alternative to timing changes to prevent detonation.

interesting........

 

[Metalguy]

>"One degree can be the differance between getting down the track or breaking the reing lands off every piston."<

Something sounds wrong here. A TF/FC engine can make 3 times more HP and not hurt the pistons (if things go right).
Yes, nitro burns slow (50-60 deg. spark lead), but it sounds like you're getting some severe pressure spikes.

OOC, why aren't you simply running as fast/quick as you can? I never had any use for "bracket racing", but don't mean to imply that it's easy.


"I'm that dog who saw a rainbow, only none of the other dogs believed me." from "Kate and Leopold"

 

[sbc438]

I have to disagree with you on that. Having had several discussions with Clay Millican (IHRA TOP FUEL Champion) in person, they do hurt parts when things go right!!!!! On a typical run they replace every rod, piston, ring, and 2-4 cylinder liners if things go right. When things go bad they degrease the back half of the car and put the other engine in. All those pretty colors you see in the exhaust flames are parts being consumes, Green is the head gasket, White is a titanium valve or an aluminum piston, yellow is the nitro burning.

We are running as fast as we can. The nitro guys are running twice the blower (14-71) and use 6 times the fuel we do in a pass. Nitro also has oxygen (in the chemical formula) in it so you can run more fuel with less air.

Running the same engine all year requires a maintence schedule and a safe tune-up. We only win $2K if we do win so replaceing a $45K engine will happen.......Never!!

8 runs replace rod bearings and mains = $200
20 runs hoan and re-ring, new valve springs = $500
40 runs replace rods and re strip the blower = $1500

We can run Top Alcohal Dragster (Pro class heads up )but the differance is our maintence ia about $7K a year. the valve spring budget on a T/AD is $30K alone....

 

[Fabrico]

Transmissions/gears can multiply torque to any value you want--that's why it's easy. But gears cannot multiply HP. The difference between torque and HP is TIME. Torque does not consider time, while HP does.
This is so elementary---.
(Metalguy)

Uh huh, how about bringing this up on another thread?

 

[automotivebreath]

The one thing that keeps popping up is engine loading. It seems everything revolves around how the engine is loaded. If it's not loaded enough we run rich and go slow. If we overload we run lean and start eating up the engine.

The answer lies in the first 15 degrees of crankshaft rotation ATDC of the combustion process. This is when engine load has the biggest impact on combustion, when cylinder pressure is rising rapidly pushing down on the piston. If the engine load is light the pressure on the piston top meets little resistance, when the engine load is heavy, resistance is high. The effect is cylinder pressure increases with engine loading.

How does this effect air/fuel ratio? The available air/fuel ratio doesn’t change with engine load. What changes is with higher cylinder pressure air/fuel separation increases. The result is more liquid fuel in the cylinder and less of the available fuel is burned. The amount of fuel that is consumed during combustion is lower; the amount of air hasn’t changed resulting in lean burn. Raw fuel is blown out of the exhaust, both increase EGT.