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An engine's ability to gain revs


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#1 just me again

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Posted 08 November 2014 - 16:46

Hi

 

Have been thinking if there is a norm for measuring the speed of wich an engine with no load can gain revs. 

 

Some times i drive a truck. It used to be a Scania P94 230hp. Really slow with 24 tonnes. But as quick as any truck when empty. Now it´s a Mercedes Antos 2643. It´s a lot faster up the small Danish hills  than the Scania, but slower than the Swede to gain speed when empty.

 

Bjørn



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#2 Canuck

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Posted 08 November 2014 - 18:03

The ability of an engine to get out of its own way.

I spent a fair amount of time dyno-testing different motorcycle exhausts on the same engine (with just fuel and ignition tuning). I had one pipe that, on the road felt much quicker than the dyno graph would suggest. You could still feel the "hole in graph" but it seemed to sail right through it to be quicker overall in hard acceleration.

I never have been able to quantify or identify that.

#3 imaginesix

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Posted 08 November 2014 - 18:57

The ability of an engine to get out of its own way.

I spent a fair amount of time dyno-testing different motorcycle exhausts on the same engine (with just fuel and ignition tuning). I had one pipe that, on the road felt much quicker than the dyno graph would suggest. You could still feel the "hole in graph" but it seemed to sail right through it to be quicker overall in hard acceleration.

I never have been able to quantify or identify that.

Note that our bums are very poor at detecting acceleration, while they are quite excellent at detecting changes in acceleration. So if you felt the pipe that was weaker on the dyno to be stronger on the road, it may have been due to a dip in the power curve (the hole) that was more pronounced and therefore offered a stronger feeling of acceleration as it climbed out of it and back onto the flatter slope of power delivery.



#4 Lee Nicolle

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Posted 08 November 2014 - 21:46

Hi

 

Have been thinking if there is a norm for measuring the speed of wich an engine with no load can gain revs. 

 

Some times i drive a truck. It used to be a Scania P94 230hp. Really slow with 24 tonnes. But as quick as any truck when empty. Now it´s a Mercedes Antos 2643. It´s a lot faster up the small Danish hills  than the Scania, but slower than the Swede to gain speed when empty.

 

Bjørn

The engines have different torque and power bands. Gearing may well have a lot to do with it too



#5 bigleagueslider

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Posted 09 November 2014 - 03:42

Just what do you mean by an engine "with no load"? An engine uncoupled at its PTO would accelerate in roatational speed at a rate that varies over the speed range. For example, it would take less power to double an unloaded engine's speed from 500rpm to 1000rpm than it would to double the same unloaded engine's speed from 1000rpm to 2000rpm. If we assume there were no mechanical stress limits on the engine components, eventually the engine would not produce enough power to overcome the increasing internal mechanical losses, and the unloaded speed of the engine would peak out.



#6 just me again

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Posted 09 November 2014 - 18:51

On an enginedyno i think you can say that you measure the engine ability to not drop revs. Wich is not the same as the time it takes the engine to gain revs with little or no load.

 

For example double decloutching. It will take less time with a light flywheel than with a heavy one.

 

Or an engine with Catalysator and a bad/cheap Ecu. If you floor it you can count the seconds before the engine picks up.

 

Bjørn



#7 Canuck

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Posted 10 November 2014 - 15:11

Note that our bums are very poor at detecting acceleration, while they are quite excellent at detecting changes in acceleration. So if you felt the pipe that was weaker on the dyno to be stronger on the road, it may have been due to a dip in the power curve (the hole) that was more pronounced and therefore offered a stronger feeling of acceleration as it climbed out of it and back onto the flatter slope of power delivery.

Curse you for pointing out the obvious answer that completely eluded me. ;)

I needed to do some acceleration tests, but failed to do so.

#8 Lee Nicolle

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Posted 11 November 2014 - 08:05

On an enginedyno i think you can say that you measure the engine ability to not drop revs. Wich is not the same as the time it takes the engine to gain revs with little or no load.

 

For example double decloutching. It will take less time with a light flywheel than with a heavy one.

 

Or an engine with Catalysator and a bad/cheap Ecu. If you floor it you can count the seconds before the engine picks up.

 

Bjørn

A low speed truck engine will always use a heavy larger diameter flywheel to keep momentum up. Coupled with a heavy and again larger diameter  clutch to transmit the torque and load capacity.

A passenger car is always a deal lighter and smaller and a road race engine [road car derived] will use as little as will drive the usually multi plate small dia clutch. And a speedway midget or sprint uses no flywheel or clutch at all. That is why they pick up revs so quick. Top level speedway sedans use a 'coupler' in the gearbox to move off, and generally use a auto flex plate to start the engine



#9 just me again

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Posted 11 November 2014 - 13:28

A low speed truck engine will always use a heavy larger diameter flywheel to keep momentum up. Coupled with a heavy and again larger diameter  clutch to transmit the torque and load capacity.

A passenger car is always a deal lighter and smaller and a road race engine [road car derived] will use as little as will drive the usually multi plate small dia clutch. And a speedway midget or sprint uses no flywheel or clutch at all. That is why they pick up revs so quick. Top level speedway sedans use a 'coupler' in the gearbox to move off, and generally use a auto flex plate to start the engine

exactly

 

But is there not a way you measure this, or is it just by feeling if your engine is good or not to pick up revs?

 

Bjørn



#10 gruntguru

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Posted 17 November 2014 - 08:57

Some times i drive a truck. It used to be a Scania P94 230hp. Really slow with 24 tonnes. But as quick as any truck when empty. Now it´s a Mercedes Antos 2643. It´s a lot faster up the small Danish hills  than the Scania, but slower than the Swede to gain speed when empty.

 

There are at least two properties that can produce or add-to the effect you describe.

 

1. The higher empty weight of the Mercedes is a factor. The higher inertia of engine moving parts is also significant in lower gears (and therefore at high engine acceleration rates).

2. At high engine acceleration rates the turbocharger will produce a significantly different boost profile (lower boost) than for steady state. This effect is generally more pronounced for larger turbochargers due to rotational inertia.

 

One easy way to measure the effect of 2. and 1b. is a timed free-rev of the engine. How long it takes to accelerate from say idle to 1500 rpm when your floor the throttle.


Edited by gruntguru, 17 November 2014 - 09:00.


#11 rory57

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Posted 17 November 2014 - 18:21

Could the differences you perceive between the two trucks be down to different ways of matching turbochargers to engines? And of course the exact way the engine controller is programmed. A laggy turbo set-up gives the impression of less power in transient conditions perhaps?

#12 just me again

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Posted 21 January 2018 - 15:41

I have been thinking on this again.

 

That there should be a measure of an engines ability to gain and loose revs.

 

The other day I drove a company car, a Ford Focus, Probably 1.0 turbo. if you accelerate in second gear to the speed limit you can go direct to 5 or 6 gear. But for the problem that the engine only drop the revs very slowly! Much slower than my own non turbo petrol car.

 

I find this annoying. A lot, but it seems there is no measure for it and therefore no way, but trying, to find out which cars have this "issue"



#13 kikiturbo2

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Posted 21 January 2018 - 23:00

it is called rotating assembly inertia. It is very evident on modern small engines that are 3 cylinder or even 2 cylinder designs. To keep things smooth they use very heavy twn mass flywheels and you end up with an engine that doesnt like to accelerate trough the rev range..



#14 just me again

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Posted 23 January 2018 - 07:05

Yes. The Ford is a 3 cylinder. But my own Peugeot is also. And does'nt have that issue.

I would think the Turbo also have it's part in the Ford's behavior!

#15 kikiturbo2

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Posted 24 January 2018 - 07:40

Yes. The Ford is a 3 cylinder. But my own Peugeot is also. And does'nt have that issue.

I would think the Turbo also have it's part in the Ford's behavior!

 

I have that peugeot 3 cyl with turbo.. :D

 

turbo has nothing to do with it.. it is all down to rotating assembly inertia and engine mapping.. Yes, some ECU's deliberately slow down the engine decel.



#16 just me again

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Posted 24 January 2018 - 10:52

Mine is without.
Maybe it's the ECU. I just feel the Ford is much slower to drop the revs than my car!

#17 Greg Locock

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Posted 24 January 2018 - 17:52

/Dropping/ revs in neutral may not be simply fuel off. There was a time, early 90s, when many cars would 'hang' at about 1800 rpm on the way down. This was for emissions.



#18 V8 Fireworks

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Posted 25 January 2018 - 11:40

 

 

The other day I drove a company car, a Ford Focus, Probably 1.0 turbo. if you accelerate in second gear to the speed limit you can go direct to 5 or 6 gear. But for the problem that the engine only drop the revs very slowly! Much slower than my own non turbo petrol car.

 

 

This is rev-hang and a characteristic of the drive by wire programming rather than the engine itself.

 

Just drive a rotary or some other hard-tuned engine with a traditional cable throttle with a ultra-lightweight flywheel with little inertia.  Those are excellent example of engine which gains and loses rpms very quickly.


Edited by V8 Fireworks, 25 January 2018 - 11:41.


#19 kikiturbo2

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Posted 25 January 2018 - 20:16

 

 

Just drive a rotary or some other hard-tuned engine with a traditional cable throttle with a ultra-lightweight flywheel with little inertia.  Those are excellent example of engine which gains and loses rpms very quickly.

 

I recently drove the new 911 R with optional lightweight flywheel.. when you switch the negine off it just dies instantly... :D



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#20 Kelpiecross

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Posted 26 January 2018 - 05:15

/Dropping/ revs in neutral may not be simply fuel off. There was a time, early 90s, when many cars would 'hang' at about 1800 rpm on the way down. This was for emissions.


It was also very annoying.

#21 V8 Fireworks

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Posted 27 January 2018 - 19:50

It was also very annoying.

 

Saving the environment is more important, right?  :)



#22 7MGTEsup

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Posted 05 February 2018 - 14:14

Saving the environment is more important, right?  :)

 

I'm not sure re instating fuel at 1800rpm is more environmentally friendly then switching fuel back on 200rpm above target idle speed.



#23 Greg Locock

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Posted 05 February 2018 - 19:17

It was,otherwise they wouldn't have done it would they?



#24 7MGTEsup

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Posted 06 February 2018 - 11:43

It was,otherwise they wouldn't have done it would they?

 

So why is the practice not used now? The vehicles I work on don't come out of DFCO till about 200rpm above target idle. No fuel injected has to be more efficient than some fuel injected. Maybe they were struggling with NOX emissions with the tip in after DFCO so they introduced the higher fuel reinstatement to try to stop excess oxygen storage on the cat brick from a long DFCO?



#25 Greg Locock

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Posted 06 February 2018 - 19:17

I don't know why. Bear in mind that in the 80s and 90s the state of the art with emissions was pretty crude, and we were all just learning (not that I've ever worked on emissions, I have done a bit of fuel/spark mapping and some other stuff) .



#26 malbear

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Posted 09 February 2018 - 07:18

My Ducati prototype would gain revs very rapidly  and scare little kids . Guys from the other side of the oval would come running thinking someone was starting a hotted up V8.

this rapid gain was due to lack of drag by the normal cam poppet valve settup. The head was an integral part of the power generation and not parasitic.. the  rotating assembly inertia was about the same as the standard 4 stroke Ducati. 



#27 427MkIV

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Posted 06 March 2018 - 16:32

I don't know if this is exactly what you're talking about, but Larry Widmer (some think he's a genius, some think he's a fraud) refers to "transient response," or an engine's ability not just to make big numbers at high rpm on a dyno but instead its ability to recover revs and power quickly after a shift/make power at the bottom of the gear. He talks about it in this post: https://speedtalk.co...hp?f=1&t=44121

 

I'm interested in knowing if any of you know of Widmer's work and your opinions of his ideas on heads and piston design, rod length, etc.


Edited by 427MkIV, 06 March 2018 - 16:36.