Fuel Consumption

[andreworiordan]

I own a 25hp X-RNLI tiller (2stroke) I was wondering what the average fuel burn would be on this type of engine, Im planning a trip away and would love to know what she will burn. I think the engine is around 2001.

Pics can be seen at;

http://quiksilver430.blogspot.com/

[ian parkes]

My yam 25 2 stroke used about 1/2 litre a mile on a large SIB or 10 litres an hour roughly.

[Poly]

Originally Posted by ian parkes

My yam 25 2 stroke used about 1/2 litre a mile on a large SIB or 10 litres an hour roughly.

I have its baby brother (20 HP - same block I think). And that sounds about right to me -for normal cruising etc... at WOT its almost double that though.

[ian parkes]

Originally Posted by Polwart

I have its baby brother (20 HP - same block I think). And that sounds about right to me -for normal cruising etc... at WOT its almost double that though.

yes i meant to say cruising which was about 18mph half throttle and WOT was about 21

WOT is the point I thought , and sold it

[Locozodiac]

The standard consumption to almost any outboard engine is 10% of it's horsepower. Example : a 25 HP 2 stroke engine will consume 2.5 gallons an hour at full throtle. It also will depend on prop being used, gasoline octane, wind, water conditions and how well you boat is inflated.

One good test is to put one gallon, maintain any specific aceleration with out any throtle variation and see hoy much time you run till the engine stops. In this way will have you gas consumtion to that particular aceleration.

Run at cruising speed and se how much time you can cruise with one gallon.

Happy Boating

[bernithebiker]

A good tip for consumption is to not use higher octane fuel than you need to. In the US alot of the gasoline is around the 90 RON mark (often lower). The higher the octane, the higher the resistance of the fuel to pre-ignite. Therefore, as long as there is no danger of pre-ignition (pinging/knocking), then a lower octane fuel will ignite easier and run smoother.

So basically, in the UK (and France), don't go using 98 instead of 95 thinking it will work 'better', as unless your outboard specifically demands it (I don't know of any that do), chances are it will work less well.

There are lots of interesting threads on this subject if anyone is interested.......

[BogMonster]

Interesting, I always thought high octane fuel was more likely to go bang, didn't realise it was the reverse. I heard of somebody here a few years ago who was running 100LL aviation fuel in a trials bike, apparently won just about everything but didn't do the engine much good!

[jwalker]

Originally Posted by bernithebiker

The higher the octane, the higher the resistance of the fuel to pre-ignite. Therefore, as long as there is no danger of pre-ignition (pinging/knocking),

I think you are confusing resistance to detonation with resistance to pre-ignition.

[bernithebiker]

Originally Posted by BogMonster

Interesting, I always thought high octane fuel was more likely to go bang, didn't realise it was the reverse. I heard of somebody here a few years ago who was running 100LL aviation fuel in a trials bike, apparently won just about everything but didn't do the engine much good!

No, high octane fuel is LESS likely to go bang.

It is a common misconception.

In the quest to extract more performance from petrol engines, desginers raise the compression ratio (how much the piston squeezes the fuel/air mixture) as high as they can. Theoretically, the higher the compression ratio, the higher the engine efficiency. But as compression increases, the higher the chance that the petrol will 'pre-ignite' (burns before the spark plug fires). This causes an irregular burn, high pressure spots, and the 'pinking' noise.

Higher octane fuels were designed to allow these higher compression engines to function reliably. They are able to resist the higher pressures and temperatures of higher tuned engines before being ignited by the spark plug.

Super tuned race engines have very high compression ratios (either by head design or by turbos, etc.) and therefore NEED high octane fuel (98 up to 130+)

If you look in your car/outboard handbook, you can find your compression ratio. Most petrol engines are in the 8 to 10 / 1 range. In a 4 cylinder, 2 litre engine, a 10/1 ratio means that the piston/cylinder goes from a max.volume of 500cc to a minimum of 50cc. Using 98 RON in such an engine will gain you nothing, and will most likely reduce efficiency, albeit by a v. small amount.

I used to have an Audi S4, one of the few cars out there to officially 'prefer' 98RON, due to turbos, etc. Although I used 98 sometimes, it ran just as well on 95, and I never noticed any pinking.

I am not in the US, but I believe it is quite difficult to buy 95 RON petrol, let alone 98.

[Locozodiac]

Originally Posted by jwalker

I think you are confusing resistance to detonation with resistance to pre-ignition.

I agree with you, confusing terms,

Quoted from a Mercury Service Manual:

Use a mayor brand of automotive unleaded gasoine with a minimum posted octane rating of 90 RON. Higher gasolines that contains fuel injector cleaners are preffered for added internal engine cleanliness. The use of higher octane gasolines will not affect the overall engine performance as long as the ignition timing and engine are correctly tuned.

I use 95/97 octane unleaded gasolines with Chevron TCW-3 / 2 stroke oil on my Tohatsu's 05-18-30 HP, and they work fine, the price difference is so small down here compared to 90 RON, so, why not use a cleaner aditive gasoline. It's absolutely true that a lower compression engine requires a lower octane gasoline and viceverse.

Happy Boating

[bernithebiker]

This is an excellent analysis of this subject;

http://www.rinconriders.com/index.cf...42&thrid=44584

[bernithebiker]

Originally Posted by Locozodiac

I agree with you, confusing terms,

Quoted from a Mercury Service Manual:

Use a mayor brand of automotive unleaded gasoine with a minimum posted octane rating of 90 RON. Higher gasolines that contains fuel injector cleaners are preffered for added internal engine cleanliness. The use of higher gasolines will not affect the overall engine performance as long as the ignition timing and engine are correctly tuned.

I use 95/97 octane unleaded gasolines with Chevron TCW-3 / 2 stroke oil on my Tohatsu's 05-18-30 HP, and they fly...

Happy Boating

Note, that they promote the use of gasolines with cleaners in them. Noone is arguing with that.

But nothing is to be GAINED, performance wise, by using higher OCTANE gasolines.

Indeed, the argument goes that potentially there is a negligible performance LOSS when using higher octane gasolines (depending on individual engine design).

It's a hard one to swallow, I know, given how the oil companies love to sell you more expensive fuel, but use a good quality 90RON with an cleaning additive and you will be just fine.

[bernithebiker]

that above link may not work so, just in case, here it is in full;

Subject: High Octane Fuel, a good read


Here is another, and my favorite! I posted this many times, and decided to make it its own thread

High Octane Fuel - Do You Really Need the "Good "Stuff?

or:

You Really Can Fool Some of the People All of the Time


"The swami has been hearing a lot of nonsense around the gas pumps these days. People are tanking up with the "good" stuff because the commercials imply that it's better for their engine. When the oil companies use superlatives like "Super", "Extra" and "High"...well it must be better, right? And of course they wouldn't be charging $0.10 - $0.20 more unless they were putting some really good stuff in there, right? Sorry...NOT!

"High Octane" is not synonymous with "good" or "better", and does not mean that it is better for your engine! And the chances are pretty good you don’t need high octane fuel in your scooter.

High-octane fuels only become necessary when your engine has a high compression ratio. It’s a very long and complicated story…that the swami will make short.

First important fact that you must accept:

All gasoline, regardless of its’ octane rating, have pretty much the same amount of energy per gallon. What!!! "Sacrilege" you say? Well, actually, some higher-octane fuels have a few LESS percent energy per gallon…so as not to argue over this small point, for the sake of this discussion we will all agree that the automotive gasoline that you buy at the pump, regardless of octane rating, has the same amount of potential energy.

Second important fact that you must accept:

Octane is NOT a measure of power but of the fuels’ resistance to ignition from heat. A higher-octane fuel, under identical combustion chamber conditions, will burn slower.

How can this be? If all of the above is true, how do we get more power out of high octane gasoline? We do, don’t we?

Well…yes we do. Here’s how:

But first you must understand "heat of compression". There is a 2,000 year old fire starting device that still amazes the swami. A length of bamboo was hollowed out leaving one end capped. A stick, about the same length as the bamboo, was whittled down until it fit snugly into the bamboo cylinder. A bit of dried grass or wood shavings were placed in the bottom of the bamboo cylinder and the snugly fitting stick was violently rammed down the bamboo tube. The heat generated from rapidly compressing the air in the tube was sufficient to ignite the tinder.

The same thing can happen in the cylinder of an engine. The piston, quickly squeezing the fuel/air mixture into a small space, can generate enough heat of compression to ignite the fuel well before the spark plug fires, with unpleasant results. If the fuel prematurely ignites while the piston is on its way up, the burning of the fuel, in conjunction with the rising piston, creates even more pressure, resulting in a violent explosion. This explosion is equivalent to hitting the top of the piston with a very large hammer. If you want to be able to see through the top of your piston, ignore those sounds that are usually called: "pre-ignition", "ping" or "engine knock". Trust me on this one; in his reckless youth, using this method, the swami turned a few pistons into paper weights.

What we really want is a very rapid burn of the fuel, not an explosion. And we want the burning of the fuel to take place while the piston is in a better position to convert this pressure into productive work, like on its way down. Think of this burning as a very fast "push" on the top of the piston. Despite the violent noises you hear from some exhaust systems, it really is a rapid push on the top of the piston making the crankshaft go around, not explosions.

So that we can ignite the fuel at exactly the right time with the spark plug, instead of from the heat of compression, they put stuff into gasoline to keep it from igniting prematurely. The more resistant the fuel is to ignition from the heat of compression, the higher its octane rating.

Are you with me so far?

Higher compression ratios = higher combustion chamber pressures = higher heat… and it is with these higher combustion chamber temperatures that the magic happens.

At higher temperatures the fuel is burned more efficiently. So, while it’s true that the higher-octane fuel does not posses any more energy than low octane fuel, the increased octane allows the extraction of more of the potential energy that has always been there. Conversely, lower compression ration engines utilize a little less of the fuel energy potential (2-4% reduction) but there is also less heat generated in the combustion process.

So how do you know if you need high-octane fuel? The swami suggests you look in the owners’ manual! Manufacturers really do want you to get the maximum efficiency out of your engine. They do their best to give a good balance between horsepower and engine life. It’s in their best interests to do so.

There is ABSOLUTELY NO BENEFIT to using a higher octane than your engine needs. The only benefit is increased profits to the oil companies that have cleverly convinced some of the public that their new "Super-Duper, Premium-High-Test, Clean-Burning, Used-By-Famous-Racing-Types-All-Around-The-World, Extra-Detergent-Laden-Keep-Your-Pipes-Clean, Extra-High-Octane" fuel is your engines’ best friend. The swami is telling you the truth, don’t listen to that talking cartoon car.

The swami hears people insisting that they got better mileage, better acceleration, and less dental plaque by switching to a high-octane fuel. The swami reminds these people that in every pharmacy is a special miracle pill that is often prescribed by doctors, it works wonders because people believe that it works wonders; it’s called a "placebo". The swami warns: never confuse faith with physics!

If you are getting pinging or knocking with what should be the correct octane for your engine, start by checking the ignition timing, also check that the spark plug is the correct heat range. For 2-strokes, check for excessive carbon build-up on the top of the piston, the carbon takes up space and increases the compression ratio.

If all is well and correct, and you still are getting knocking, then try the next higher octane. You won’t go faster, you won’t go farther, but you will prevent an unsightly hole in your piston.

This subject is a whole lot more complicated than the swami wants to bother with. If you are curious to know more, put some of these words into your search engine and enjoy the education:""

Antiknock Index

Octane

Stoichiometric Combustion

Thermal Efficiency

Flame Front

Highest Useful Compression Ratio

Compression Ratio

Placebo

[bernithebiker]

Originally Posted by jwalker

I think you are confusing resistance to detonation with resistance to pre-ignition.

You are right, strictly speaking, it is detonation.

[v666]

Originally Posted by bernithebiker

But nothing is to be GAINED, performance wise, by using higher OCTANE gasolines.

Unless you have an engine management system that can adjust the timing to make use of the higher octane fuel. Or you have had your chip re-mapped.

[bernithebiker]

Originally Posted by v666

Unless you have an engine management system that can adjust the timing to make use of the higher octane fuel. Or you have had your chip re-mapped.

Indeed.

[jwalker]

Originally Posted by v666

Unless you have an engine management system that can adjust the timing to make use of the higher octane fuel.

Are you suggesting that the engine can produce extra power from higher octane fuel by adjusting the ignition timing?

[Poly]

Originally Posted by bernithebiker

In the US alot of the gasoline is around the 90 RON mark (often lower). The higher the octane, the higher the resistance of the fuel to pre-ignite. Therefore, as long as there is no danger of pre-ignition (pinging/knocking)

Except that in the USA the Octane rating used at the pump is not the RON number as it is in the UK. It is measured differently. UK standard road petrol (95 RON) would probably be sold in the US as 91 Octane (the exact number depends on the blend - they are not directly convertable like say US gals to UK gals). Similarly the standard road gasoline in the US which is sold as 87 octane would be sold in the Europe as something like 92 RON.

then a lower octane fuel will ignite easier and run smoother.

It would be true to say it takes more energy to initiate combustion in a higher octane fuel mix than a low one - but the energy from the spark is so high in comparison to either that this is irrelevant.

[v666]

Originally Posted by jwalker

Are you suggesting that the engine can produce extra power from higher octane fuel by adjusting the ignition timing?

Yes.
Using a higher octane fuel will allow more ignition advance, this will produce more HP.

[jwalker]

Originally Posted by v666

Yes.
Using a higher octane fuel will allow more ignition advance, this will produce more HP.

If that were true, then you could do exactly that with a 'normal' engine.