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  #1  
Old 15 Nov 2010
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altitude and octane

I just read this concerning driving a 2008 Honda efi petrol car in Peruvian Andes, but relevant anywhere high with a modern engine running on low-octane:

"... however at high elevations you can get away with lower octane than at sea level"

(ie: the crap 84 RON fuel in rural Peru worked OK at 4000m)

Does that make sense? Would lower octane petrol be improved by a richer mixture, or was their modern efi and lambda merely getting round it, just as a modern efi bike would do?

Will a modern bike run better at high altitude on low octane fuel than at low altitude on the same fuel? I would have thought mixture (rich at alt) is unrelated to octane (pre-ignition).

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Old 15 Nov 2010
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yes

- Does that make sense?
Yes

- Would lower octane petrol be improved by a richer mixture, or was their modern efi and lambda merely getting round it, just as a modern efi bike would do?
A1: No, richness does not alter a fuel's octane rating.
A2: If you mean "will it reduce the frequency/likelihood of detonation due to a less than optimal octane rating (all other things being equal)" then the answer is yes. Richer mixtures burn cooler (to a point). Retarding the ignition will help too. Avoiding high load/low rpm.

- Will a modern bike run better at high altitude on low octane fuel than at low altitude on the same fuel?
A: Fundametally, there is less oxygen available as the altitude increases so less fuel can be burnt per cycle. Modern bikes with lambda sensors can manage the risk of detonation (retard ignition) so the modern bike will run better at low altitude vs the slightly increased detonation-resistance that altitude gives you.

- I would have thought mixture (rich at alt) is unrelated to octane (pre-ignition).
A: Do you mean detonation (Pre-ignition is initiated by an ignition source other than the spark e.g. hot spots in the combustion chamber
). True - mixture is not related to octane. Mixture is related to denonation, especially in the context of low octane, which is what I think you mean.
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Last edited by edteamslr; 15 Nov 2010 at 17:51.
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  #3  
Old 15 Nov 2010
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So ECU variable timing manages the detonation (thanks for clarification) - getting round low octane at any alt I guess.
And efi with lambda ought to manage the mixture as air pressure varies.

Richer (with cooler running benefits) may not be relevant I now realise, if the best efi/lambda manages to always make a perfect mixture.
Or at 4000m+ can efi only be expected to be a lot better than a carb, but still richer than optimal?

What do efi riders find with the mpg at very high alts (3500+); the same or a little worse? I never got anywhere near that height on mine.

Q - Will a modern bike run better at high altitude? ..........
A - ... so the modern bike will run better at low altitude


So that's a no, or did you mean high alt?

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Old 15 Nov 2010
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thoughts

So ECU variable timing manages the detonation (thanks for clarification) - getting round low octane at any alt I guess.
And efi with lambda ought to manage the mixture as air pressure varies.
> EFI with an air pressure sensor (throttle position and temperature sensor) manages the mixture alongside which the lambda sensor provides the feedback on the EFI's chosen mixture. The lambda sensor is not critical for mixture regulation (open loop systems don't use the sensor, closed systems do).

Richer (with cooler running benefits) may not be relevant I now realise, if the best efi/lambda manages to always make a perfect mixture.
Or at 4000m+ can efi only be expected to be a lot better than a carb, but still richer than optimal?
>Firstly, "richer" is a relative term that represents a compromise over the optimum stoichiometric mixture to achieve a particular drivability/performance aim and cooler relates to the environment of the cylinder where combustion is taking place not the engine as a whole. There isn't some intrinsic flaw with carbs and altitude. It's the transition between different altitudes without mechanical intervention that makes FI so appropriate and convenient.

Q - Will a modern bike run better at high altitude? ..........
A - ... so the modern bike will run better at low altitude
So that's a no, or did you mean high alt?
> I thought that was obvious, if the modern bike runs better (power etc) at low altitude on poor octane fuel then it can't be better at high altitude.

Thermodynamics was never one of my strengths but where motorcycles are concerned it is fascinating.
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Old 16 Nov 2010
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In my experience, I have always suffered from increased fuel consumption at altitude.

Whether this is due to the fact that I spend a lot of time climbing (which requires more power resulting in an increase in twisting the right wrist) vs. increased consumption due to altitude, I cannot say.
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Old 16 Nov 2010
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Light aeroplanes are great with this stuff ... they run carbs, often up to 15,000 feet without turbos and well over 20,000 feet with turbos. And the pilot controls the mixture.

The difference with an aeroplane and a motorcycle engine is the carb in the aeroplane is designed to have easy to adjust mixture carb ... and an exhaust gas temp (EGT) gauge to tell when mixture is at optimal power. A bike engine has a fixed mixture carb.

In a carbed aeroplane, you would always adjust the mixture to reach maximum EGT. At that point you have the most efficient combustion ... the perfect ratio of fuel and oxygen. Then we would richen it a bit to come off maximum EGT. Those old engines were designed to run a bit rich ... I was always told because the leaded aviation fuel (even now they dont have unleaded avgas ... the best you get is low lead avgas) helped lubricate the upper cylinder ... thus you run the engine a touch rich.

Modern EFI bike and car engines are designed with more upper cylinder lubrication and dont need to run rich like the 1950s aeroplane engines in most light aircraft. Some run leaner than others. BMW bikes for example seem to run their engines very lean (still on the rich side of peak EGT, but much closer than other manufacturers) ... that means they can get better economy, but exhaust headers will often glow red when idling on a black night.

As mentioned by Ed, many EFI / ECU modules also incorporate anti detonation sensors that allow the engine to deal with low octane fuel. Its the lambda sensors that deal with adjusting the richness of the mixture as the bike climbs in atmosphere. The superiority of EFI at altitude is simply because the ECU can adjust how much fuel is squirted for any given amount of airflow thru the throttle body, thus maintaining optimal mixture. (by sensing how much unburnt oxygen is still present in the exhaust gases)

Getting back to what I suspect was the original question ... does altitude have an impact on detonation resistance?

In theory no ... all other things being equal. Its the anti-knock sensors that retard the timing that do that. However ... in the real world, yes altitude does have an impact. But not directly. Its not because the air is thinner. Its because it is colder. Detonation is caused by the heat building up in the air fuel mixture on the compression stroke. Once it reaches a certain temperature, it will detonate, or explode. (Diesel engine can run much higher compression because diesel detonates at much higher temperature). On a turboed engine, where the air is partially compressed (and therefore heated) even before it enters the cylinder, they run intercoolers to reduce the temp of the partially compressed air back to room temp - precisely for that reason, to cool the air, which will increase detonation resistance and therefore allow for higher compression, and therefore more power / economy. Similar to this, drag racing and other very high performance vehicles (including jet aircraft) running very high compression often use water injection ... water is injected into the cylinder or inlet manifold to cool the air/fuel mix under load, to give detonation resistance.

In a normal bike or car engine ... when driving from the atacama desert at +40 degrees to the altiplano at 0 degrees, on the same fuel, there will be increased detonation resistance, because the colder air at altitude will not reach the same temperature when compressed 12:1 or whatever the compression ration of the engine is.

At least thats my understanding of it

My own personal experience, is at high altitude, above 4000 metres, your use of throttle is lighter ... because you are inevitably on windy mountain roads, not flooring it on long straights. Ergo you are less likely to experience detonation in any case.

On my particular EFI bike (and from talking to other with the same engine) going into altitude significantly IMPROVES fuel economy ... by 20-30%. The exact opposite of what you would expect from a carbed bike. It was something I was not expecting to see. Riding across Tajikstan saw my economy improve from about 4.2 l/100km to just over 3 l/100km. While other friends with similar riding styles and 650 cc crbed bikes have reported going from 4.8 l/100km to 6l/100 km while in Tajikistan.
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Last edited by colebatch; 16 Nov 2010 at 09:57.
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  #7  
Old 16 Nov 2010
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Not that I have any idea, but could improved fuel consumption at altitude have anything to do with lower air resistance due to a thinner atmosphere?


I lived in Nairobi many years ago, 1700 metres high. Whenever I drove to the coast at Mombasa the car seemed to perform faster. The locals said this was due to increased air pressure and oxgen at sea level. A sort of mild turbo/supercharging effect.
I can't remember any effect on fuel consumption, but fuel cost was not an issue in those days!.
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Old 17 Nov 2010
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My 2 cents

I'd say that altitude does have an influence on detonation, in the sense that the compression end temperature is reduced in the following way:

At altitude, the ambient air pressure is lower (I noticed on my GPS with barometer that @ 5000 m air pressure is down to 550 mbar, so only 55% of sea level ambient!). Starting out the compression stroke at this lower pressure also reduces the compression end pressure, and consequently the compression end temperature. It is this end temperature that determines whether detonation occurs. Lower temperature = less chance of detonation.

The other effect of course is the lower initial temperature of the inlet air. Lower initial temperature = lower end temprature.

So yes, low octane fuel is better suited to higher altitudes.

The reduction in fuel consumption also puzzles me a bit, but I think that Tony gave us at least part of the explanation; standing up on your bike at higher altitudes is definitely easier than at sea level: decreased air resistance.
Another one might be that you are actually using your engine closer to it's maximum potential (for the conditions, that is). As indicated above, your proud 1200 cc Beamer is reduced to a humbler 55% x 1200 = 660 cc bike, comparing air mass flow between sea level and 5000 meter.
You'll be opening the throttle further to experience the same sensation of accelaration / power, which is beneficial because the throttle valve is actually quite a restriction and eats up power. And, any type of equipment running at 75% iso 35% of it's potential is usually more efficient.
All assuming of course that the carburettor / injection has been adjusted for the lower air pressure.

A word about detonation / octane:
Under normal conditions, being constant speed, you are far away from the limit at which detonation occurs.
The fuel's octane rating is designed to avoid detonation under all conditions. The most adverse being high load, high ambient temperature, low revs. That is when detonation occurs. So, if at all you notice detonation, just shift down one gear or reduce your speed / accelaration and you'll be fine. This should work fine except on very highly tuned, very high compression engines.
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Old 18 Nov 2010
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Quote:
Originally Posted by aukeboss View Post
I'd say that altitude does have an influence on detonation, in the sense that the compression end temperature is reduced in the following way:

At altitude, the ambient air pressure is lower (I noticed on my GPS with barometer that @ 5000 m air pressure is down to 550 mbar, so only 55% of sea level ambient!). Starting out the compression stroke at this lower pressure also reduces the compression end pressure, and consequently the compression end temperature. It is this end temperature that determines whether detonation occurs. Lower temperature = less chance of detonation.

The other effect of course is the lower initial temperature of the inlet air. Lower initial temperature = lower end temprature.
I am not sure about the lower starting pressure having any impact on the final compressed temperature.

Two given volumes of air (one at seal level 1 ATM, and another at altitude 0.5 ATM) both at say 20 degrees C, are compressed 12 times ...

They should both reach the same temperature under compression, because both are compressed 12 times. It doesnt matter that one is at 12 ATM and one is at 6 ATM.

If you look at the high altitude sample, that began at 20C and 0.5 ATM, ... as it gets compressed thru 1 ATM, it will be already much hotter than the uncompressed 1 ATM sample which we know is at 20C. So the two samples will not have the same temperature at the same pressure - Delta temp is not a result of absolute pressure, delta temp is a result of delta pressure.

So while the two samples will not have the same temperatures at the same pressures, they will however have the same temperate at the same amounts of compression (the change in pressure). What affects the change in temperature is the amount of compression, not the initial starting pressure.

Ergo, 0.5 ATM compressed 12 times from 20C will reach the same temperature as 1.0 ATM compressed 12 times from 20C.

So as far as I can see, the benefit to detonation resistance is just from lower temperature ... as you said, lower temp in, lower temp under compression.
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Old 18 Nov 2010
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Auto-ignition is frequently interchanged with detonation on forums (ignore pre-ignition, which is ignition from hot spots/non-spark source within the combustion chamber).

Detonation in gasoline engines is the auto-ignition of the END GAS i.e. the part of the charge that has not yet been consumed by the flame front AFTER the spark initiates it. When the flame front reaction expands it raises the temp and pressure of the charge outside this front which then reacts rapidly. Please update your terminology accordingly.

Next,
"Ergo, 0.5 ATM compressed 12 times from 20C will reach the same temperature as 1.0 ATM compressed 12 times from 20C."

Two things here - 1) the temperature may be the same but the density of the air in the two example volumes will be different (pV=nRT)
Therefore, 2) when you compress a volume of gas you are actually doing "work" on it and providing you are assuming an ideal gas, the formula for a reversible adiabatic process relies on V being the 'molar volume' which in your two cases would not be the same, hence the resultant two temperature deltas would be different.

The pressure increase is more than a simple 12:1 compression ratio suggests because the gas is not only compressed, but the work done to compress the gas has also heated the gas and the hotter gas will have a greater pressure even if the volume had not changed.

Just for your interest, compressing 1 litre of air at room temp/pressure in the ratio of 10:1 increases the temperature from 27C to 477C.
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Old 18 Nov 2010
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This has sparked my brain cell into life, 0.5atm @ 12:1= 6atm at full compression, 1.0atm @12:1= 12atm at full comp. I'm still trying to drag up Boyles law from the far distant past and apply it to the puzzle.....Also low atm effects the "lift" of the fuel up the jet/s in a carb.
I once saw the head off a small two stroke racer which had been used at altitude (in South Africa, I think) it had a massive squish band and v. small combustion chamber to address the first point.
Though compression ratio stays constant, the actual compressed pressure drops with altitude, so pinking or pre-ignition is less likely to occur(I think?)
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Old 18 Nov 2010
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One post later...

Compression ratio is about mathmatical volume - not pressure or mass..

1 ATM is 14.7psi or 101.325 kN/m2 = pressure

1ATM becomes 24.5ATM with 10:1 compression ratio at sea-level/room temp/air.

Quote:
Originally Posted by edteamslr View Post
The pressure increase is more than a simple 12:1 compression ratio suggests because the gas is not only compressed, but the work done to compress the gas has also heated the gas and the hotter gas will have a greater pressure even if the volume had not changed.
Quote:
Originally Posted by oothef View Post
Though compression ratio stays constant, the actual compressed pressure drops with altitude, so pinking or pre-ignition is less likely to occur(I think?)
If pinking (detonation) was (just) occuring at your low altitude example then it would be less likely at higher altitudes (on the same fuel).
Pre-ignition is different but may be helped by the lower temps at higher altitude.
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Old 18 Nov 2010
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So does 0.5 atm become 12.25 atm at 10:1 (or there about)
There's much less initial pressure for a flame front (or two) to work against to create detonation.
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Old 18 Nov 2010
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Yep, it's all about the temperature and pressure in the cylinder.

So, because altitude reduces the mass of air in the cylinder for otherwise identical conditions, the temperature and pressure at the end of the (approximately adiabatic) compression, and indeed throughout combustion, are lower.

As a result, you get less knock - in the same way as you get less knock at sea level by running with a part-closed throttle (which also reduces the mass of air in the cylinder and hence reduces temperatures and pressures).

As a secondary effect, the air in the cylinder is also cooled by the evaporation of fuel, and a carburettor is a simple device which doses fuel by volume of air, not mass of air. So at altitude, where the same volume of air is less mass of air, the temperature of this smaller charge is reduced more by the evaporation of the fuel than at sea level (where the same fuel would be cooling more mass of air). So again, the temperature during compression and combustion is lower still (which also reduces the pressure even more) and the tendency to knock is reduced again. So yes, the effect of reduced knock at altitude is increased by the undesirable over-richening of the mixture, but a fuel injection system which keeps the mixture right at altitude will still have improved knock resistance at altitude for the primary reason above...

Add another effect that ambient temperature is lower at altitude, and the charge in your cylinder is therefore cooler to start with even before the fuel's evaporated into it, and the temperatures and pressures in all engines are even lower still! The same effects cause problems in diesel engines at altitude, and old diesel cars will often emit a haze of unburnt fuel out of the exhaust when descending mountains, because the temperatures and pressures in the cylinders are too low to burn the fuel - on the way up the mountain the engine's working harder and therefore hotter, so it carries on working fine...

So in short, as the octane rating of a fuel is it's resistance to knock, an engine designed to make the very most of 92 RON at sea level will be fine on 80 RON if you take it to a high enough altitude.

And yes, you're right, I am an insufferable engine geek, but someone's got to be or the world'd run out of engines...
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Old 18 Nov 2010
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This is fantastic stuff. And has my brain working overtime to keep up Its 25 years since I last looked at Physics

Quote:
Originally Posted by edteamslr View Post

1ATM becomes 24.5ATM with 10:1 compression ratio at sea-level/room temp/air.
I had forgotten about the heating of the air under compression impacting additionally on the pressure ...

So let me see if I understand this correctly ... temp being constant, 1 ATM compressed 10 times will be at 10 ATM ... but the fact that temp isnt constant (due to rising pressure) and it would reach 24.5 ATM means the Kelvin temp of that air has risen 2.45 times? Is that right?

But that would give me a temp of 462 C, rather than the 477 C you mentioned above (initial air temp 27 C)
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