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balthazar

RPM vs. emissions

32 posts in this topic

More idle blather from my 'commuting home' brain...

An average motorist (let's call him 'Randy'), not particularly an 'enthusiast', has a choice of two to make for his new midsize sedan.

Nearly everything else is equal and the prices are right. But one is advertised and in fact has a high-revving 6-cylinder with a redline of 8000. The other is a torquey 6 with a redline of 5500.

Since our man Randy is a bleeding heart ecologist, he asks himself:


Given the same owner, roads traveled and driving techniques, would the car that requires on average 1500 more RPM to deliver the same level of performance... that extra RPM would have to in fact deliver a greater volume of emissions, right?
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Well Randy, apparently the slower reving 6's no longer lead the torque game. However this brings us to the reason why the slower reving engines get better fuel sipping at givin normal driving power band, while the speed racers running constantly above 6 grand dont. To further add to the confusion how many of those high output high RPM engines ever see that 7000 before they are reduced to started cars for teenage boys ? At that point will they be more or less environmentaly friendly ?
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razor: I have no idea what you said above. Do you have an answer to the question on the table??

carguy: Nothing's been 'proved' at all, but I would like to hear how --if possible-- a higher-spinning engine would NOT emit a greater volume of pollutants (either way pretty minor in today's cars... just wondering).
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razor: I have no idea what you said above. Do you have an answer to the question on the table??


Thats OK neither do I. Something like the higher reving high HP V6's now have the same or more torque than the slower reving ones have.

Then something about how all that high reving business has little to do with everyday driving, until the boy racers get the car in its dying years at which point it wont be very environmentally friendly anyhow.

Yes the higher an engine revs the more fuel it needs to burn thereby producing more polution givin the effeciency of all engines are the same.

However at normal driving RPM ranges it makes little difference.
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I don't understand the question. I know which is more fuel efficient, but quicker and for efficient? Probably the high-revving one. Simply because it has a wider band down low and doesn't put out as many pollutants at low RPMs and is fast once you shift down...?
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In the past, I would agree that a higher-revving engine (e.g: Honda engines) would just be moving the emisions higher up in the powerband than a less rev-happy engine with gobs of low-end torque (i.e: any Domestic V8). But a lot of rev-happy, OHC engines now have Variable Valve Timing, which does a better job of distributing power, so that you don't have to rev them so high to get useable torque. Sure, the torque may still peak @ 4,000 RPM, but, with VVT, the torque curve is a lot flatter. Edited by Petra
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Higher revving would mean less efficient, because there would be more friction at those rpms. The engine with a lower peak torque would also not need to rev as high to get going with traffic, like my Grand Prix with 280 torque at low rpms, I only need to rev to 2000 to get going with the flow of traffic, while a car with no torque until 5000rpms would need to rev above 3500, which uses more gas to get going with traffic.
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Higher revving would mean less efficient, because there would be more friction at those rpms. The engine with a lower peak torque would also not need to rev as high to get going with traffic, like my Grand Prix with 280 torque at low rpms, I only need to rev to 2000 to get going with the flow of traffic, while a car with no torque until 5000rpms would need to rev above 3500, which uses more gas to get going with traffic.

[post="15466"]<{POST_SNAPBACK}>[/post]


but higher RPM's also means more moving mass = higher inertia => friction does not play as big a role. BTW, friction only depends on how tight the fit is between the piston and the cylinder, not how fast the piston is travelling.

Also... this tidbit of info comes to mind... the Ferrari F40 (I believe) has almost 0 emissions at 4,000 rpm.
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Perhaps we should look back to the begining of the 20th century. Back to the long throw cranks and enormous flywheels of the hit and miss single lungers. Now they went along ways on one combustion cycle. Could that have been considered DOD in its most primitive form ? ;-)
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In the end the truth will prove how once again the Japanese have been pulling wool over our eyes. As I mentioned recently it's pretty funny how many kids will pour tons of money into a cramped, FWD, four banger Civic and after spending a fortune impress a of us Muscle car guys. But when questioned why even bother to put all that money into a POS fwd four banger they say fro practicality & fuel economy.... it's more practical to drive a little four banger civic. But the truth is that a Civic CRX wiht $20,000 worth of mods that runs 11.9 in the 1/4 mile is still just a cramped POS but it now get's 12mpg. My 1968 Camaro could be a 11.9 car and still get 12 mpg. Buit I still have a much nicer, classic car wiht more comfort and the durrability/affordability of RWD and cheap /simple pushrod V8. I remember when the SRT4s came out and I was very impressed wiht their timeslips only to hear FIRST hand froma guy at N.E. Dragway that he was getting about 10mpg. That's my $0.02 on the subject. Sorry I don't have more scientific info for you Balthy. Where's the token Nerd? So it;s like WHY bother? I'll keep my RWD, cheap to buy cheap to fix cheap to upgrade PUshrod powered car. Edited by Sixty8panther
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Well, if this is thread that is supposed to convince me tha OHV is in some way better than OHC... I don't see the point. Each has their highs and lows... It really comes down to preferences. Do you prefer high or low end power? I happen to prefer high end... However there is another thing that helps decide between the two... Overall refinement. Look at any LS series V8... they are perfect. More refined than some OHC V8s. This is excellent, however when it comes to V6s... they lack it. I know the General has the resources to do it, but it hasn't happened. I don't know why. If they could make an OHV V6 that is the equivilant the LS OHV V8s... They'd have a winner.
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I've been playing around trip computers on several cars, and all of them get 1 MPG when I floor it. A smaller displacement engine reigns supreme once you've reached the speed you want to maintain. According to my Passat's trip computer, I get about 9 MPG accelerating from 0-40, but 60 MPG crusing at 40. It averages out to be ~23 MPG city. As far as RPM vs. emissions (fuel consumption), I'm not an engineer, but don't turbos force more air (and fuel) at low RPMs? And you'll be hard pressed to find any commuter car that revs freely to 8000 rpm. Most DOHC V-6's make maximum power at 6000 rpm, while larger OHV V-6 make it at 5500.
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RE: redlines: It was a hypothetical example, emp; not literal. And it's not the torque peak per say, but the torque curve and how well the car moves & feels like it's moving.
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Answer: "It depends on the design details more than the behaviour of the engine". If the engines are equally efficient, then whether it produces more torque at lower rpms or less torque at higher rpms is irrelevant. If they both produce the same power then the fuel burned and emmisions prodiuced will be similar. Combustion efficiency is more important. In low load/low rpm conditions 2V engines can be (but aren't necessarily) more efficient. This is why (together with cost) Honda is still designing new 2V engines and why port deactivation is so popular. In high rpm/high load conditions 3-5V heads help (but are not sufficient) to burn more fuel cleanly to produce more power. Aside from the ease in incorporating multi-valve heads, cam placement and number is irrelevant. Any engine can be designed to be very fuel-efficient at lower rpms, by simply using less fuel and producing less torque. As engine speed increases, power will also increase, until torque falls off faster than the engine speed increases, but fuel-burn and emmisions will also increase with the in rpm. If low-rpm power is adequate for the application, then you can offer a combination of low rpm fuel-efficiency and high-rpm power, when the driver desires. A VVT system can change between a low rpm torque regime and a high-rpm torque regime, but fuel-efficiency isn't the only reason such systems are used. They can be optimised purely for maximum power, minimal fuel use, a balance of both, or alternate between the two extremes. Theoretically timing and lift can be altered independently of rpm, offering either efficiency or maximum torque at any given engine speed depending on the conditions.
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Answer: "It depends on the design details more than the behaviour of the engine".

If the engines are equally efficient, then whether it produces more torque at lower rpms or less torque at higher rpms is irrelevant. If they both produce the same power then the fuel burned and emmisions prodiuced will be similar. Combustion efficiency is more important. In low load/low rpm conditions 2V engines can be (but aren't necessarily) more efficient. This is why (together with cost) Honda is still designing new 2V engines and why port deactivation is so popular. In high rpm/high load conditions 3-5V heads help (but are not sufficient) to burn more fuel cleanly to produce more power. Aside from the ease in incorporating multi-valve heads, cam placement and number is irrelevant. Any engine can be designed to be very fuel-efficient at lower rpms, by simply using less fuel and producing less torque. As engine speed increases, power will also increase, until torque falls off faster than the engine speed increases, but fuel-burn and emmisions will also increase with the in rpm. If low-rpm power is adequate for the application, then you can offer a combination of low rpm fuel-efficiency and high-rpm power, when the driver desires. A VVT system can change between a low rpm torque regime and a high-rpm torque regime, but fuel-efficiency isn't the only reason such systems are used. They can be optimised purely for maximum power, minimal fuel use, a balance of both, or alternate between the two extremes. Theoretically timing and lift can be altered independently of rpm, offering either efficiency or maximum torque at any given engine speed depending on the conditions.

[post="16008"]<{POST_SNAPBACK}>[/post]


Huh???

I'm a retired engineer, and I don't follow you! Say it in English, please.

Your first statement is right on the head- "It depends on the design details more than the behaviour of the engine."

When engines are designed and tested, two parameters are used that usually
don't see the light og day for the consumer enthusiast.
One is the BEMP, which is Brake Mean Effective Pressure, and is the measure of
how efficient the engine design is at any snapshot point.
A corollary to this is the fuel consumption measured in lbs./hr.
This factor wraps up the design of fuel delivery and burn efficiency.
Another factor that you don't normally see is pumping losses. This is power requirement necessary turn the internal engine parts caused by the internal
friction of the engine. This relates to surface areas and swept volume of the moving parts.
The basic question is more complex than you gave it credit for, and not enough
info was stated for a valid comparison.

Remember and internal combustion engine is nothing more than an air pump!
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aright so say that my van revs in normal driving between 2000 and 4500 rpms, no joke, if i added some power to this engine and got it reving lower but with the same imput, would that increase or decrease emissions? or how about economy?
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razor: I have no idea what you said above. Do you have an answer to the question on the table??

carguy: Nothing's been 'proved' at all, but I would like to hear how --if possible-- a higher-spinning engine would NOT emit a greater volume of pollutants (either way pretty minor in today's cars... just wondering).

[post="15425"]<{POST_SNAPBACK}>[/post]

You seem to think I am on the other side of the argument them I am. I (It seems to you) have the opinion that higher revving Engines dont pullute any more then slower revving ones. Were you got this idea I dont know but I agree with the very first post. Its only logical even with VVT and other TECH that the higher an Engine revs to make its power the more pollutants it will deliver!
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No; I agree with you: the faster a given engine spins, the more combustion cycles per minute (RPMs) and thusly more emissions. I did ask if anyone could disprove this... so far: nada. The problem with emission testing with regards to this thread is that it is usually performed at moderate & steady RPMs. This is the only hard data available yet does not reflect how a high-revving auto is often driven. I am not aware of any testing to measure the cumulative volume of motor vehicle exhaust, but I'm sure it would be interesting.
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No; I agree with you: the faster a given engine spins, the more combustion cycles per minute (RPMs) and thusly more emissions. I did ask if anyone could disprove this... so far: nada.

The problem with emission testing with regards to this thread is that it is usually performed at moderate & steady RPMs. This is the only hard data available yet does not reflect how a high-revving auto is often driven. I am not aware of any testing to measure the cumulative volume of motor vehicle exhaust, but I'm sure it would be interesting.

[post="17016"]<{POST_SNAPBACK}>[/post]


The EPA doesn't test at specific RPMs, but rather "whatever it takes" the car to perform the testing routine, which specifies the speed it must travel during each second. This includes idling and brisk acceleration.

I don't know if theoretically, more RPMs = more fuel burned, but certainly there are engines that make most of their power at higher RPMs AND get excellent fuel economy.
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>>"The EPA doesn't test at specific RPMs..."<<
I was referring to readily available data via state inspection stations; the EPA/manufacturer emissions data is not released publically as far as I know (beyond classifications: LEV, SLEV, etc).

>>"...certainly there are engines that make most of their power at higher RPMs AND get excellent fuel economy."<<
The questions remain: would said engines get equal fuel economy if their power was developed at lower RPMs AND also emit less emissions?
Is/ how is high RPMs related to fuel economy and emissions?
Are these engines getting "excellent fuel economy" at 80% of redline, or during relatively steady-speed EPA tests?
What are individual engine's emission volumes?
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>>"The EPA doesn't test at specific RPMs..."<<
I was referring to readily available data via state inspection stations; the EPA/manufacturer emissions data is not released publically as far as I know (beyond classifications: LEV, SLEV, etc).

>>"...certainly there are engines that make most of their power at higher RPMs AND get excellent fuel economy."<<
The questions remain: would said engines get equal fuel economy if their power was developed at lower RPMs AND also emit less emissions?
Is/ how is high RPMs related to fuel economy and emissions?
Are these engines getting "excellent fuel economy" at 80% of redline, or during relatively steady-speed EPA tests?
What are individual engine's emission volumes?

[post="17127"]<{POST_SNAPBACK}>[/post]


Fuel consumption is a direct function of CO2 emissions; the EPA measures the amount of carbon released during the simulation and converts the mathematical equivalent into MPGs. Re: "would said engines get equal fuel economy if their power was developed at lower RPMs AND also emit less emissions?", such an engine would not emit less emissions...
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The EPA doesn't test at specific RPMs, but rather "whatever it takes" the car to perform the testing routine, which specifies the speed it must travel during each second. This includes idling and brisk acceleration.

[post="17093"]<{POST_SNAPBACK}>[/post]


How "brisk" can the acceleration be or how much power does it take on those big rollers. NOt a lot of rolling resistance and the car is not actually travelign therefore pushing mass and air. The testing is flawed too.

I don't know if theoretically, more RPMs = more fuel burned, but certainly there are engines that make most of their power at higher RPMs AND get excellent fuel economy.


It's called a Northstar 4.6L 32 valve V8. :lol: :)

Seriously 19MPG average in all driving conditions and yet it'll bring you to 128mph quicker than a Cop can swallow a Jelly Donut. Edited by Sixty8panther
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>>"such an engine would not emit less emissions"<<

Maybe I'm not making my point well, empowah.
If a mid-sized car makes 200 HP at 6500 RPM, it will require a certain level of throttle (& therefore RPMs) to deliver the desired level of acceleration in normal driving... ie- more RPMs to develop the required power.

If that very same car developed 200 HP at 4500 RPM, that exact same desired level of acceleration would be met using 2000 less RPMs... and 2000 RPMs less of emissions.

Every other revolution per minute of the engine produces 1 exhaust pulse per cylinder. If Car X averages --say-- 3500 RPMs during acceleration, the quantity of emissions is less than one averaging --say-- 4500.

And if the EPA calculates MPG based on a conversion of CO2 emissions, that agency is even more useless than I thought.
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