# engine braking calculation

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### #1 mariner

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Posted 02 February 2013 - 10:05

I am putting together a small talk on US sprint car racing for my local car club, as I think about what to say a thought has come to me - I have no real idea how to calculate engine braking effect.

Sprint cars have pretty weird brakes - like only three sometimes, but they can slow down real quick when needed. I ASSUME there is a lot of engine braking available because they have 410 cu. in ( 6.7 litre) engine with 16 : 1 compression. They only weigh 1,400lb/640kg so the engine braking should be good.

I can do aceleration from surplus torque to weight etc. but I've never ever thought how you calculate engine braking. As the engines have cam overlap I wonder if the effective CR is less than 16:1 with the fuel shut off but I dont really know.

Anybody have any ideas how to estimate/ gues the engine braking?

There are some references but I suspect they are just assumptions

http://www.mchenryso...nputtables1.htm

http://www.racer.nl/...m#enginebraking

As an indicator the WoO sprint cars are geared to do about 130 mph at 8,000 rpm. I think the rear end ratio is about 4:1 and they have just one gear.

### #2 Lee Nicolle

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Posted 02 February 2013 - 11:26

I am putting together a small talk on US sprint car racing for my local car club, as I think about what to say a thought has come to me - I have no real idea how to calculate engine braking effect.

Sprint cars have pretty weird brakes - like only three sometimes, but they can slow down real quick when needed. I ASSUME there is a lot of engine braking available because they have 410 cu. in ( 6.7 litre) engine with 16 : 1 compression. They only weigh 1,400lb/640kg so the engine braking should be good.

I can do aceleration from surplus torque to weight etc. but I've never ever thought how you calculate engine braking. As the engines have cam overlap I wonder if the effective CR is less than 16:1 with the fuel shut off but I dont really know.

Anybody have any ideas how to estimate/ gues the engine braking?

There are some references but I suspect they are just assumptions

http://www.mchenryso...nputtables1.htm

http://www.racer.nl/...m#enginebraking

As an indicator the WoO sprint cars are geared to do about 130 mph at 8,000 rpm. I think the rear end ratio is about 4:1 and they have just one gear.

410 Sprintcar engines are about 14-1 I feel, maybe a bit less. Gearing is whatever the car is geared at, usually in the 6s or high 5s on the US Larger ovals. And Sprinters really do not stop, they slow down is all. One of the reasons they crash so often,, but really they cannot stop anyway, turn left with so much stagger, an 18" wide right rear couples with a couple of 8" fronts.
A typical brake set up on one with brakes will be 12" vented iron rotor under the seat on the L/R.With a eg Superlite 4 spot calliper Maybe an 10-11" ultralight with a tiny twin piston callper outboard R/R. And similar to R/R on the L/F and very maybe literally a Kart calliper on the R/F
A lot have far worse/ smaller than this, A scaloped 11" single alloy rotor L/R and a tiny brake L/F. Literally the brakes are so they do not hit the infield officials at 20mph.
The fixation with weight is huge, everything goes including brakes.
Though the thinking drivers have more what I originally outlined. On dirt ovals you never want much front brake, usually r/f are used only on flatter tracks, sometimes with a tap or line pressure valve. a l/f brake can lock going into a corner making the car push wide. But the l/r brake should be a decent one as it does most of the work, and pulls the car into the corners.
My old classic Supermod has only rear drum brakes with a bigger cylinder on the l/r. I tried a l/f brake and it caused the car to push so it was removed again, though that is what it used in period. Though for a few laps they are probably morre effective than some modern cars! And weigh more too.

Edited by Lee Nicolle, 02 February 2013 - 11:29.

### #3 J. Edlund

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Posted 03 February 2013 - 00:28

Engine braking is frictional losses + pumping losses (piston pulling against inlet manifold vacuum); basically the torque required to drive the engine with the throttles closed. The impact of compression ratio on engine braking would be marginal at best.

### #4 bigleagueslider

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Posted 03 February 2013 - 05:08

Sprint cars on a short dirt oval mostly scrub off speed by sliding in the turns. Any type of oval dirt track racing, whether sprint cars or flat-track/speedway motorcycles, is all about how much speed you can carry through the turns by sliding, and then how quickly you can put the power back down on the straightaways. With oval dirt track racing, there is little concern for any type of braking.

Regardless, the max amount of braking from a sprint car on a dirt track, whether produced from brakes or the engine, will be limited by the traction between the tires and track surface. So what you should really be concerned about is the traction capacity of the tires, and not so much the braking torque of the engine.

### #5 Magoo

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Posted 03 February 2013 - 07:13

What Edlund said. Or why diesels have zero natural engine braking despite extreme CR: no throttle. CR = ER so you can think of compression as like a valve spring. It takes work to compress it but you get it back.

With constant-flow FI, sprinters require high idle speed/high throttle angle to prevent stalling; also, no flywheel or clutch, direct drive, low rotating inertia. The driver pitches the car sideways and then paddles the throttle to manage the car's attitude through the corner.

Two examples, winged and unwinged:

### #6 JimboJones

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Posted 03 February 2013 - 12:24

In answer to the original question, you cannot guess/estimate engine braking. You measure it on a dyno just like you do the WOT performance, and it can be mapped just the same way too, down to a minimum torque which is a function of friction and throttle position (pumping losses) as already mentioned.

### #7 mariner

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Posted 03 February 2013 - 17:48

thanks for the answer above to my question. I guess a run down test in gear would also measure the engine braking.

Magoo's videos gives me an excuse to add one with GPS and G data.

### #8 Lee Nicolle

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Posted 03 February 2013 - 22:56

Sprint cars on a short dirt oval mostly scrub off speed by sliding in the turns. Any type of oval dirt track racing, whether sprint cars or flat-track/speedway motorcycles, is all about how much speed you can carry through the turns by sliding, and then how quickly you can put the power back down on the straightaways. With oval dirt track racing, there is little concern for any type of braking.

Regardless, the max amount of braking from a sprint car on a dirt track, whether produced from brakes or the engine, will be limited by the traction between the tires and track surface. So what you should really be concerned about is the traction capacity of the tires, and not so much the braking torque of the engine.

Agreed. Sideways scrubs off more speed than brakes on a sprinter.
Speedway solos have no brakes at all, nor do sidecars. And there has been the occasional 2 stroke solo and many 2 stroke sidecars. And not a great deal of engine braking on a 2 stroke.

### #9 Lee Nicolle

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Posted 03 February 2013 - 23:11

What Edlund said. Or why diesels have zero natural engine braking despite extreme CR: no throttle. CR = ER so you can think of compression as like a valve spring. It takes work to compress it but you get it back.

With constant-flow FI, sprinters require high idle speed/high throttle angle to prevent stalling; also, no flywheel or clutch, direct drive, low rotating inertia. The driver pitches the car sideways and then paddles the throttle to manage the car's attitude through the corner.

Two examples, winged and unwinged:

It shows the difference betwen a hooky tack and a slick track too. The outlaw car is half throttle cranked sideways every where whereas the USAC car is as tight as. And seemingly could use some brake.
But the wing on the outlaw car is still letting it into the corners faster and theoretically off too. Though as I said half throttle!!
The Outlaw car also shows why front brakes are not of great use as the l/f is steering at the fence most of the time.

Edited by Lee Nicolle, 03 February 2013 - 23:15.

### #10 gruntguru

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Posted 04 February 2013 - 04:42

It is difficult to find empirical friction values but FMEP of 1.5 bar at max power might be a reasonable guess. In addition closing the throttles would produce a MAXIMUM retardation MEP of 1 bar (at the impossible intake pressore of zero absolute - a more reasonable value would be 400 kPa giving a retardation MEP of 0.6 bar) for a total of 2.1 bar. For a 410 ci engine this converts to a retardation torque of 113 N.m (83 lb ft).

The above is very "seat of the pants" and "back of the envelope" so I welcome any corrections.

Edited by gruntguru, 04 February 2013 - 04:48.

### #11 Greg Locock

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Posted 04 February 2013 - 05:10

It is difficult to find empirical friction values but FMEP of 1.5 bar at max power might be a reasonable guess. In addition closing the throttles would produce a MAXIMUM retardation MEP of 1 bar (at the impossible intake pressore of zero absolute - a more reasonable value would be 400 kPa giving a retardation MEP of 0.6 bar) for a total of 2.1 bar. For a 410 ci engine this converts to a retardation torque of 113 N.m (83 lb ft).

The above is very "seat of the pants" and "back of the envelope" so I welcome any corrections.

You have trod where angels fear to. At 0% throttle there is no net flow so I can't see how you get 1 bar of pmep. There will be a bit as the air rushes backwards and forwards across the exhaust valve. As you suggest it might average out around half a bar.

### #12 gruntguru

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Posted 04 February 2013 - 06:16

You have trod where angels fear to. At 0% throttle there is no net flow so I can't see how you get 1 bar of pmep. There will be a bit as the air rushes backwards and forwards across the exhaust valve. As you suggest it might average out around half a bar.

I have assumed with zero intake pressure:
1. Intake stroke at zero pressure - work required = 1 Bar x displacement
2. Compression stroke at zero pressure - work required = -1 Bar x displacement
3. Power stroke at zero pressure - work required = 1 Bar x displacement
4. Exhaust stroke at 1 Bar pressure - work required = Zero (Exhasut valve opens @ BDC and fills cylinder, pressure stays at 1 Bar throughout exhasut stroke)

Nett work from crankshaft to gases = 1 - 1 + 1 + 0 = 1

### #13 Greg Locock

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Posted 04 February 2013 - 20:00

I've asked at the other place, I certainly used to run engines on the dyno with no fuel and with the throttle in various positions, but that's a diagnostic for NVH, not a performance test.

Oh I've just remembered testing this subjectively in the late lamented Corona. 5000 rpm in second, ignition off, modulating throttle as the car slows. No apparent effect. OK, it was a lousy test but it shows that there may not be much in it.

Edited by Greg Locock, 04 February 2013 - 20:02.

### #14 Kelpiecross

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Posted 05 February 2013 - 06:53

I've asked at the other place, I certainly used to run engines on the dyno with no fuel and with the throttle in various positions, but that's a diagnostic for NVH, not a performance test.

Oh I've just remembered testing this subjectively in the late lamented Corona. 5000 rpm in second, ignition off, modulating throttle as the car slows. No apparent effect. OK, it was a lousy test but it shows that there may not be much in it.

I have done much the same test but down a steep hill - I have also noted (much to my surprise) the same as you - very little, if any at all, effect. It would be interesting to do the same downhill test but with the sparkplugs taken out. I have the feeling that the car would fairly whizz down the hill as if it were in neutral.

Edited by Kelpiecross, 05 February 2013 - 06:54.

### #15 Lee Nicolle

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Posted 05 February 2013 - 07:50

All the figures and formulas in the world wont show the retardation of a 13 1/2-1 410 with a 6.5 diff ratio when they jump off the throttle. It does nearly drag the wheels initially. A 500cc solo engine is the same, though they have more compression and a flywheel of sorts. A midget may be more impressive to as they have a bit more comp and far smaller tyres, 84" v 106" right rears. Far less centrifigual mass.

### #16 gruntguru

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Posted 05 February 2013 - 08:51

Oh I've just remembered testing this subjectively in the late lamented Corona. 5000 rpm in second, ignition off, modulating throttle as the car slows. No apparent effect. OK, it was a lousy test but it shows that there may not be much in it.

Not surprising when you think about it.
Guesstimate engine braking for your test: 2.5 Bar FMEP at WOT, perhaps 3.0 at closed throttle 20" vacuum. Difference = 0.5 Bar.

Compare this to normal throttle response at 5,000 RPM: minus 2.5 Bar BMEP at closed throttle, plus 10 bar at WOT. Difference = 12.5 Bar.