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??

 

[Metalguy]

How do you not fully load a dragster engine? W/O "liquid dynamite" it should either be at idle or WFO.

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

 

[sbc438]

The best way to describe the way it works is kind of like a battery.

Let's say you have a battery with 900 amps avalible, if you wire in a 100W light bulb you will only pull 8.3 amps. Now if you dead short the battery you will pull all 900 amps. The amount of power released is directly related to the load.

Now our engine has a potental to make 1950hp (ish) but if we gear it low enough and make our torque through gear reduction we are not "loading" the engine. i.e. like a battery we will not pull all of the HP out that is possibile. So we can't burn all the fuel avalible and end up rich. The limit with a race car is you can't "dead short it (battery analogy)" cause you most likely don't have the traction to put all the power to the ground.

This is a concept I've been trying to get my head around for a year now. I think I'm finally on the way to understanding it.....I think....i 'm just looking for some confermation.

 

[CCycle]

Here is the way it seems to me.
Please point out if this is wrong.
At a given throttle setting, the only thing that determines power out put is how much torque you are pulling. If you load it lightly the RPM goes up and the pistons tend to outrun the fuel burn. Since the pistons are moving fast the pressure drops fast and the fuel tends to burn slower.
If you use more torque, rpm drops and the fuel has more time to burn. Pressure stays higher so fuel burn rate is faster. Fuel is burned up and if there is excess oxidizer it will go after the metal parts.

 

[ivymike]

You're not going to get any here. The combustion process doesn't stop partway through just because the crankshaft is unloaded. If the engine is unloaded, the crankshaft accelerates. I'm having trouble following your discussion of torque, loading, and power. How long has it been since you were degreeded?

 

[Metalguy]

"The limit with a race car is you can't "dead short it (battery analogy)" cause you most likely don't have the traction to put all the power to the ground."

True for Top Fuel/Funny Car. But 2000 HP ain't diddley these days. Forget torque--drag racing is all about HP. Torque is easy to get--HP isn't.


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

 

[sbc438]

Ivymike,

It's been 8+ years since I took my last Physics class but I wasn't trying to be exact in terminology. When it's all said and done this system must follow the laws of physics and conservation of energy. With all things constant (gearing, fuel tune-up, tire size, weight, .....) when load on the engine is varied (usually by changing the % of slip in the torque converter or clutch) combustion changes i.e. going rich or lean. I'm looking for that missing link....cause in reality this is what happens.

This is the reason why when you see a Top Fuelers hase the tires it will drop cylinders and push raw fuel out the exhaust.

Metalguy,

We are working with much less tire and a much harder hit off the line as a % of avalible power @ launch so 2000hp is still a challange. We also are running to a dial in so we have to repeate our run with-in .005 sec. each time to be competative. Fuelers just need to get to the end of the track first...


HP is a function of Torque, It's all related and there's many ways to get to the other end of the asphault (5 speeds, 2 speeds, glider clutches, ect). What I've found doing this is it's a fine ballancing act of controling the load on the engine, and adding or removing fuel as load (traction) changes. The other side is as load stays constant and other (atmospheric) conditions change you have to add or remove fuel to keep the power output equal to the loading. This is all in the effort to be repeatable.

For example last weekend our 1/8 mile times were as follows over three days in varying conditions from 52 F to 83F

Qualifying
4.120 sec.
4.113
4.095

Eliminations
4.109 sec.
4.115
4.111

 

[ivymike]

I'm looking for that missing link....cause in reality this is what happens.

How about looking at the air/fuel ratio going into the cylinder for a start? Does your carburetor adequately control the mixture when engine speed changes rapidly? Does it adequately control the mixture throughout the rev range?

 

[SomptingGuy]

If the engine speed changes dramatically through a cycle, the whole compression/combustion business is going to be compromised. When loaded the changes will be less.

 

[sbc438]

We run a posative pressure injection system where the pump pressure is variable with engine rpm. The main controling parts of the system are the barrel valve activated buy throttle position, main bypass jet (used to richen or lean mixtures) and a high speed lean out in the form of a pressure relif that is shimmed for adjustments. The whole system is bench flowed and set up and small adjustments are made from there.

We monitor mixture by looking at EGT readings and can make assumptions from there. This system is very sensative but reliable. We have reached points this season where we added more Boost and more fuel and had little or no gain due to overpowering the torque converter (too much slip not letting the engine loaded) which caused a rich condition. At this point we added another 9psi boost, no fuel and had no gains and were still rich. This was a point A/F changes made no differance so we tightened the converter from 17% slip to 2.5% (to much). With this combination we were 800 rpm lower going through the traps and had to reduce the main bypass jet .010" (richened considerably).
Now we can burn the fuel but we don't have enough power to spin the blower up fast enough after the shift (converter's dragging the engine down. Unfortunatly we are running the blower @ 69.5% overdrive (14,000rpm) and if we go any higher it will do real bad things!!!!!!!


Somethingguy,

We come off the line @ 2000rpm go to 8700rpm in 2.1.5sec it pulls down to about 6200rpm and climbs back to 8200rpm over the next 4.2 sec.

I'm not really looking for a definative YES/NO answer but more like "your in the ball park"

There are infinant variables in what we are doing and that's what makes it fun. When you qualify at noon and run 6.600 sec and win the race at 1am and run a 6.597 pass you have concored the variables.

 

[WilliamH]

I'm no expert on drag racing.

However, it seems to me that the fuel system on this engine is totally RPM dependent and not load sensitive (no manifold pressure feedback). Is this true? If so, engine load and fuel mixture are seldom going to be "correct".

Maybe I am missing something here?

 

[sbc438]

In what we're doing feedback systems that also control "anything" on the engine are not allowed. So we use the known pump curve and flows of injectors (just a atomizing nozzel) along with RPM to match the fuel needs to the load. Tuneing is done by bypassing fuel and releving pressure with springs, shims, and bypass orifices. This is a very Mechanicly simple, infanitly adjustable, and extreamly complex to figue out (ecspecially when conditions are always changing). Oh, ya, it's extreamly fail safe that's why EFI is not allowd (we'd also be making enough power to turn the earth backwards if we did).

 

[Fabrico]

It's all about tuning. And tuning involves a lot more than just air and fuel. It includes masses, traction, weigt transfer, etc etc. The whole car is tuned for a specific set of circumstances.

As far as the engine itself goes, a loose example is something already mentioned about how long it would take to blow up in neutral. It would take x.x seconds. With some re-tuning, and expensive testing, it could likely be done significantly faster.

Saying torque is easy or something to be forgotten about is absurd. It's the only thing that can perform work.

 

[patprimmer]

I think it is simply that with a higher load, the engine accelerates more slowly and as a consequence spends more time in the rpm range that produces the highest cylinder pressures. The higher cylinder pressure also transfers more heat to the castings. The higher cylinder pressures requires richer mixture to cool things down and avoid detonation. To rich results in to much cooling which results in misfires.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[CCycle]

Stay with us sbc438 I'm still trying to understand.

"We come off the line @ 2000rpm go to 8700rpm in 2.1.sec"

I assume you start at low rpm to limit engine power and keep it on the traction edge up to 2.1 sec
.
"In 2.1 sec it pulls down to about 6200rpm and climbs back to 8200rpm over the next 4.2 sec"

So something happens at 2.1 sec and you go into a more or less constant rpm mode. Probably also during this latter portion you move from traction limited to power limited. Evidentially the engine develops max power at approx 7 to 8 Krpm.

Does this rich condition you speak of associate with over speed of the engine?
And the lean condition come at undesirably low rpm?

 

[patprimmer]

CCycle

His high speed lean out should compensate for changes in VE due to blower speed. These are positive displacement blowers, so VE is controlled by the blower to a much greater degree than it is by the engine. Low VE of the engine simply causes boost levels to rise so the same mass of air is pumped through the engine.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Metalguy]

Fabrico,

>"Saying torque is easy or something to be forgotten about is absurd. It's the only thing that can perform work."<

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---.

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

 

[mattsooty]

I think that the most fundamental problem you have is that of fuelling control - simple as that. Maybe I am over simplifying it but lets be honest this post wouldnt exist if you sorted the fuelling out....Forget about torque/HP debates

MS

 

[sbc438]

Ccycle,

My referance to engine speeds wasto "loosely" describe how we accelerate throught the 1/4 mile. To clearify our launch rpm is 2K the go WOT and accelerate to 87000rpm where we shift to second gear and that pulls the engine speed down to 6200rpm and we accelerate from there to around 8200rpm at the finnish line all @ WOT.

Mattsooty,

We have a handle on the fuel setup but in what we do we arn't really looking to always have the perfect A/F ratio. We atually use fuel to cool the air charge, Blower, and as patprimer said to keep the engine from detonating to an extent. The thing I can't get my head around which I've seen several times, is change only one variable such as gear, tire dia, or torque converter efficancy (all effecting engine load) to lessen the load on the engine and we will have to take fuel away over the entire RPM range to run the same 1/4 mile time.

So if you went to the simpelest of analysis, our time rate consumption of fuel is lower with less load on the engine. What complcates it more for me (understanding it) is with a gear change you increase engine rpm and you would think it would need even more fuel (more power strokes for a given time period). If we just changed our final gear from a 4.10 to a 4.29 we will loose 200 degrees F in EGT so we pull fuel out to get EGT's back up to 1100 F

SO, less load = more RPM @ the same elapsed time = less fuel consumed as a function of time. This just seems completely backwards to me unless I explain it using conservation of energy laws.

Using that I can rationalize backwards:

Less load and the same EGT will require less fuel for
combustion But the higher RPM and parasitic frictional losses consume more energy. Now the question is, what are the quanatative values that will ballance this equation???

Is the change in load equal to the increased parasitic loss in the engine and more power strokes needed due to increased RPM?? This is where I think my answer is!!!!

 

[CCycle]

Many thanks for the several posts in this thread.
However, we still have not been able to identify a mechanism that explains the phenomena sbc438 observes.

My feeling is that it has to do with fuel burn rate and that the burn rate can change significantly.
The pistons in that big engine can under certain conditions out run the flame front.

We learn in Chemistry 101 that temperature greatly effects reaction rate.
In fact at room temperature, a temperature increase of 18 degrees will typically double reaction rate. So a temperature change of 5% absolute will typically cause a reaction rate change of 2x.
Granted combustion temperatures are much higher but can't we assume a small temperature change will cause significant fuel burn rate change?

The two things that control pressure and temperature in the cylinder are flame rate and expansion rate. Flame front speed is not independent. It is critically tied to the temperature. So we can expect that a relative small change in rpm can have a large effect on the amount of fuel burned.