altitude and octane

[Chris Scott]

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).

Chris S

[edteamslr]

- 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.

[Chris Scott]

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?

Chris S

[edteamslr]

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.

[farqhuar]

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.

[colebatch]

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.