Which two stroke engines is most economical?

All things equal, which engine would use the less fuel at a given speed – 15hp or 25hp two strokes with standard props supplied by manufacturer?

Boat in question; a plywood flat bottom fishing punt – 4.5m LOA and max Beam 1.995m (max = 1.7m WLB) with tapered blunt bow and some rocker aft to reduce wetted surface on the plane. Boat displace 423kg (930lbs) fully loaded with crew, 25hp engine, fuel etc.

The 15hp should be ample to put the loaded boat on the plane but 25hp would be an advantage to have a bit of power in reserve and more “relaxed” cruising mode not running at large throttle openings as opposed to the 15hp.
Lets say one run at about 15 knots speed range which engine would use less fuel on the same boat?
I know at WOT the 25 sips much more than the 15, but at the mentioned cruising speeds the 25hp would be barely ticking over with its larger pitched prop than the 15hp going at the same speed, and I assume would use less fuel under such conditions than its smaller brethren or am I missing something…

 

Once i had an 18 ft 1/2cab with an 85hp on it. While that engine was in the shop i fitted a 50 and my fuel consumption increased a lot. This was cruising around 18 knots. The 50 was just working to hard.

 

Two strokes are efficient when they run on their optimium power band.

15 or 25 ? Hmm... my instinct say that the 25 would be more efficient at 75 percent throttle than a 15 at full throthle.

Do some googling...many of the older 2 stroke tests on sites like.. boat test .com .. are still on the internet. They give GPH against rpm figures for many motors.

Also consider that many of the 15 and 25 hp motors were identical blocks with different tune...higher rpm

 

Almost all engines are more efficient at less than maximum output.

 

Yes but aren't most more efficient close to full throttle? That's true for diesels but w gas engines at lower loads too much heat is lost. Diesels and gas engines are very close to the same at full throttle.

Due to the ignition advance and personal experience outboards seem to do their best w about half throttle because that's where ignition advance is max or near maximum and throttle opening is still small. Above that point the engine intake noise becomes more of a roar and fuel consumption ramps up quickly. So does power but I think consumption outpaces power.

For the OP I would say if you have the 15 & 25hp engines on a boat that required full (or near full) throttle for the 15hp engine the 25 should be more efficient. If the 25 operates at half to 2/3rds throttle it should be best but if the 15hp operates at 1/2 to 2/3rds throttle to push the boat it should be best and the 25 will suffer from excessive heat loss w the much larger engine parts (like cylinder and piston crown). Sea water cooled engines suffer terribly from heat loss at very low power settings.

Just my opinion. You will note that some of the above is contradictory but the 2 stroke OB is unique w it's cold sea water cooling and it's ignition advance curve. On many older OBs most of the throttle is controlled by ignition advance alone. So I agree w the OP that the 15 should burn less fuel but only if it didn't need to work too hard. Another variable that could work into this question is the gear ratio in the lower unit. The lower gear ratio (higher number numerically) would get the efficiency nod.

 

Steele

 

bigger is always better doing less work and keep the rpms down

most 2 stroke out boards are best run at about half throttle of just above once you open them up there econamy goes out the window !!!
All engines 2 stroke or 4 stroke have a rpms where they run efficently and at there best . over then they become less efficent . from 3/4 throttle to wide open little is gained certionly not econamy . i have a 115 hp on a 14.6 boat and at 3000 to 3500 its uses less fuel that the 75 hp that i had before because its only doing a fraction of the work . at 6400 its absolutly flying and i only ever get there if its flat calm and a tail wind !! thrilling to drive and uses a tank of fuel very quickly !!

 

A larger engine also means that you can "overprop", providing the same top speed as the smaller engine and even more efficiency gain due to the reduced throttle plate restriction. Lower rpm, higher load is best for economy.

 

Conventional two stroke engines are horribly inefficient. Conventional carburated 2 strokes mix fuel and oil and induct that mixture through the crankcase. The reason they are inefficient is that to scavenge them effectively, a lot of incoming charge goes out the exhaust port. Consequently, they waste a lot of fuel and are dirty emissions wise since all of that unburned fuel goes out the exhaust stack.

There are some newer 2 strokes like the Mercury Optimax series that use the Orbital Engine company direct injection process. In these engines the fuel (along with a shot of high pressure air to help atomize the fuel) is injected AFTER the exhaust port closes. The net result is that when the scavenging occurs only air (with a bit of oil) goes out the exhaust port and the engine is as efficient as a four stroke. Another advantage is that with the direct injection system, you only have oil and air in the crankcase, and that means that you get a lot better lubrication If you want an efficient two stroke you need to look at an engine with direct injection.

Steel5 has missed a couple of key points in his comparison of four stroke and two stroke engines. The first is that the scavenging air is compressed in the crankcase (or by an outside blower), so there's work being done on that air that isn't recovered, and that is work that must be subtracted from the cycle. The two cycle is doing intake, compression power and exhaust, but it's doing the intake and exhaust at the same time. That is a blessing and a curse since it ultimately limits how much air you can process through the engine, and how much power you can get. And btw a four stroke fires every other revolution, not every fourth revolution. In a four stroke there's one stroke every half revolution.

Secondly while it sounds like you should get twice as much work from a two cycle engine because it fires on each revolution, you have to look at the port timing to understand what's really going on. A typical high speed two cycle engine opens the ports at about 90 degrees of crankshaft angle after top dead center. So while it fires twice as often, the effective displacement is only half as much, since the ports are open for almost 1/2 of each revolution (or more if it's a racing engine). A two stroke will have a better volumetric efficiency, because of the pressurized inlet and exhaust tuning, so you can get more power out of a given displacement from a 2 stroke, but it isn't anywhere close to the two times you would think you are going to get because it fires twice as often.

I'm a big fan of 2 strokes, but the realities are that they don't get anywhere near twice the power per unit of displacement because of how long the ports have to be open to scavenge the cylinder. The higher the speed the more limited they are because it takes a finite amount of time to scavenge the cylinder. Two strokes are a lot lighter, since there aren't any valves, valve gear and all that clap trap stuff, so the power to weight ratio is always going to be a lot better than a four stroke.

I think that 2 stroke direct injected engines are the future of two strokes and that's what you should be looking for if you want something that is efficient.

 

Two strokes do deliver.

Honda stubbornly tried to race Grand Prix motorcycles for many years. I'm guessing 12 to 15 years. Every motorcycle was 2 stroke except the Honda's. In all those years they NEVER won a race. They even had an engine w oval or elliptical bores and pistons. Can you imagine what that must have cost to build and race?

In GP MC racing 2 strokes rule .... literally.

 

I am not as familiar with two stroke engines, but I second Yellowjacket's observations that they are not fuel efficient at all, but are simple, light and compact for their power-out put.

The maximum efficiency on any engine occurs at the max torque rpm because that is where the max volumetric efficiency occurs. So you have to look at the power/torque curve of the engine, and try to get a prop/gearing combo that will have the engine rpms match the peak torque rpm at your cruising condition.

Typically max efficiency also occurs at wide open throttle at the max torque rpm, at least on a 4 stroke engine. Usually if max hp occurs at 5000 rpm, max torque will occur at about 3000 rpm, and that would be the optimum cruising rpm. Because of the problem with efficient scavenging rpm for the 2-stroke this will likely occur slightly differently.

I would suspect on Grad Prix motorcycle racing they have a max displacement, so the two stroke will have more power for the same displacement engine, even if less efficient. If Honda had lobbied for a rule change for the 4-stroke to allow more displacement it might have leveled the rules, but than the 4-stroke bikes would be heavier with a much larger and more complex engine. Interesting how almost all street bikes have gone 4 stroke, and many to water cooling. this produces a much more smooth and efficient engine, better for long distance riding.

 

2 stroke diesel ship engines are the most efficient if your talking converting fuel to power and these days the largest are also fully computerised including the operation of the exhaust valves
( makes you wonder if its worth talking a second look at a detroit with full electronic control?)

 

the big old 2 stroke diesels you mean?

 

Approximately, but not exactly. For example, here we see a typical engine where maximum efficiency occurs at less than max load (ie, at reduced VE).

http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_(BSFC)_Maps#Saturn_1.9L_DOHC

 

the biggest and most modern ships engines you can buy are all 2 strokes