Jetski's lose 30% of their fuel? Eh?

I got this in my mail today, just before I deleted my e-mail from their bloody NPCA (National Parks Campaign something) list for extreme abuse of e-mail accounts (Do I really need 4 different form messages per day? :wink: )

The only cite I found on yahoo was… you guessed it, from those selfsame people. And the link is dead.

It seems a little… odd to me that there would be any vehicle that dumped 30% of its fuel into the environment. That would be extremely inefficient, and indicative of poor design. But is it true at all? Is the fuel supposed to be evaporating and “gassing” out of the engine?".

Who could this mysterious and unattributed California expert be, and how could that ratio (1hour Jetski > 100,000 miles possibly be correct? That is, if I calculate right, 1 J-H (Jetski-Hour) > 1666.7 hours in a car, assuming your drivng exactly at 60 miles per hour, burning 2500 gallons of gas if fuel efficiency = 40mi/gallon. of which need be true at all. I suppose it could be due to a lack of any filtering on the Jetski, but if you are comparing different types of emissions, how do you compare them at all?

Metric for you ferriners:
100,000 miles = 160934 kilometers
2500 gallons = 9463 liters
60 miles/hour = 96.5 klicks/hour

Jet-skis (more properly “personal watercraft”) have mostly two-stroke engines (though I think there are a few four-stroke designs out there). 2-strokes are remarkably less efficient, but I’m not sure about the “30%” claim or even the 100,000 mile claim you quote. That seems to be just a LITTLE stretch of the facts.

However, jet skis are operated at full-throttle a LOT more than cars, so there is a factor to consider. It would, though, be much better to compare jet-skis to boats, which also run at high power settings a lot.

I can’t tell you if personal watercraft (PWCs – JetSkis are by Kawasaki, Waverunners are from Yamaha, Sea-Doos are from Bombardier…) really lose 30% of their fuel, but I rode two-stroke motorcycles when I was a kid. I had a Yamaha 250 Enduro (2-stk.) that would get about 35mpg. My Yamaha XJ600 (4-stk.) gets about 50mpg. True, there are 17 years of engine improvements between them, but 2-strokes are less efficient. That’s why they’ve been banned from further production for road use – too many emissions.

Two-stroke engines have a different lubrication system from four-strokes. A 4-stk. engine has oil that is circulated with a pump and used over and over. Two-strokers use premix (i.e., you mix the gas and oil in the can) or oil injection (from a reservoir) and the oil is only used once. A 4-stk. engine uses very little oil. A 2-stk. engine needs its oil reservoir topped off periodically. So yeah, you’re dumping “fuel” in a way. The oil is used once and then thrown out with the exhaust gasses.

A 4-stk. engine takes in the fuel-air mixture on the first stroke, compresses it on the second stroke, ignites it on the third, and exhausts it on the fourth. A 2-stk. engine combines the compression and exhaust into the upward “stroke” and the power/intake into the downward stroke. Since there is some overlap, there is inefficiency as some of the fuel/air mix goes unburnt out the exhaust.

But 2-stk. engines have advantages: No valves means they’re simpler and cheaper to make. They’re also lighter than 4-stk. engines and put out much more power for their size. That’s why you see them in small vehicles like PWCs, experimental aircraft and, formerly, motorcyles. (There may be some 2-stk. motorcycles still being sold for off-road use and under 100cc. I know that some European racing motorcycles use 2-stk. engines and that’s why you won’t see any on your local freeway.)

30% fuel loss? Seems high. But since the compression and exhaust strokes are combined, you are venting some unburned fuel.
FWIW, Honda is making a 4-stk. PWC.

Okay, here’s a link that explains how two-stroke engines work.

I was skeptical, but oddly enough, the guy seems to be right about the 30% figure. From Montana State gov: (

The site talks about snow-mobiles, but I assume the facts for 2-strokes would also for personal watercraft.

‘Do Motorboat Engines Cause Water Pollution?’

A little more on clean boating:

I’d say 30 percent is an upper end figure, and as most jet-skis are newer, they probably under that, but in the ballpark.

Might as well ban lawnmowers, trimmers, chainsaws, etc,etc,etc,

I’ve been using 2-stroke engines in radio-controlled vehicles, mopeds, and lawn equipment for years, and like anything else, it depends on the shape of the equipment. A properly tuned 2-stroke will waste probably closer to 5-15% of its fuel out the exhaust. Also, many modern engines use “tuned pipes” which ram some of the wasted fuel that comes out of the exhaust back into the cylinder at the end of the intake/exhaust stroke.

I’d say that a 2-stroke’s power efficiency (makes more power per liter or of volume than a 4-stroke) does make up somewhat for its wasteful fuel efficiency–you can use a smaller 2-stroke than 4-stroke for the same application.

Originally posted by Philster
**Might as well ban lawnmowers, trimmers, chainsaws, etc,etc,etc,**Or just 2-stroke engines above a certain size.

Most lawnmowers already use 4-stroke engines (Lawn-Boy is the only one I know of that even makes a 2-stroke engine nowadays). Some trimmers have four stroke engines, though they are the exception rather than the rule. At any rate, minimizing the use of two-strokes will have a positive effect. The EPA would probably let the very small ones (such as chainsaws, trimmers, edgers, etc.) slide because they are used less, 4-strokes are impractical at that size, and the total fuel consumption is pretty small for an engine that small anyway.

Erm, just to clear up the above:

Philster: “Might as well ban lawnmowers, trimmers, chainsaws, etc,etc,etc”

Beeblebrox: “Or just 2-strokes above a certain size”


Thanks for all the info. Ths really clarifies the situation.

Agreed, but I think most personal watercraft do not have the room for a proper tuned pipe. I have seen the same issue with small aircraft.

I live on a small inland all sports lake. This past weekend there must have been at least 40 - 50 water bikes on the water. By the end of the day there was a thick haze hanging above the water. Yuck! Jetskis are definitely big polluters!

[nitpick]Of course, if it’s metric for “ferriners,” you should probably use the spelling that the rest of the world uses: kilometres and litres. Only in the U.S. do you see kilometers and liters. And, I think the convention is that “klicks” means km/h (I vaguely remember a GQ thread on this sometime back).[/nitpick]

Are 2-stroke engines necessarily worse pollutors than 4-stroke? Generally, that’s true, at least in commercial applications. I remember someone talking about Honda’s “radical combustion” 2 stroke engines (a cite which notes Honda claims a 50% reduction in hydrocarbon emissions). Anyone have thoughts about 2-stroke research?

I’ve taken a look at Cerowyn’s link nad can’t help thinking that it is Honda’s spoiler tactics, since the company who has developed the direct injection 2-stroke is Australian, and has licensed it to Aprilia who have turned it into a production vehicle.

This vehicle is a 50cc scooter and its been on sale a couple of years now.

Here is Aprilia’s take on it.

I had heard this weird figure too and spent some time trying to figure out if it was true (“eight hours of use of a two-stroke engine is equivalent to 100,000 miles in a car” or something like that) and mostly came up on a dead-end. Some groups had done measurements and calculations I assumed were correct but none identified which engine and which car and which emissions.

Anyway, just to add something of substance here instead of a me-too, there’s a difference between polluting less and higher efficiency.

Efficiency is essentially energy in versus usable energy out. When fuel is “consumed” a certain amount of energy is released per unit volume. By “consumed,” in the case of gasoline, I guess that means converted to its most basic oxidized molecular form … maybe some C02, H20, NO, and some other junk.

Polluting less is, arguably, making less “bad” exhaust. C02 and H20 are generally considered least bad, CO and NO bad, and unburnt gasoline, various carcinogens, and unburnt oil are the worst.

Cars are less efficient than 2-cycle motorcycles but also pollute less. They have engines that–I believe–convert more of the gas into heat than horsepower and they have catylitic converters which essentially finish buring the exhaust gasses … plus a bunch of other environmental controls. 2-cycle engines are more efficient but also spew a bunch of crap into the environment (including a lot of unburnt gas–even in Johnny L.A.'s link above, it’s obvious that without valves, some of the unburnt gas gets out. The question is, now much?)


Hmmph—I knew “Jetson” wasn’t their real name!