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Does an engine run wide open use same amount of fuel at light load and at full load?

I recall some experiments made by bomber pilots in WW II. On the long flights to Japan and back bombers often ran out of fuel and ditched. There was some success in pitching the props to drop manifold pressure to some “magic” level while the rpms were kept at some determined level to reduce fuel consumption . That kind of experimentation may fall in line with your question @Rick. Maybe I can find that information.

"There have been no governors on any two stroke equipment that I have worked on. "
I believe that in two strokes without an efficient valve train to maximize air intake a precise times, RPMs are self limiting. Long before the mechanical components are driven to self destruct, a two stroke will run out of breath. It’s the nature of the beast which is key to it’s low end torque in allowing twice the power strokes per given given rpm, but it also contributes to it’s inefficiency as RPMs climb. For this reason, lumping two stroke and four strokes in the same performance basket is a mistake. Even if they are both technicality ICE motors, they differ too much to make generalizations of one apply too appropriately to the other.

Yes, on the two stroke engines, the rapidly falling volumetric efficiency at increasing rpm combined with the exponentially increasing power needed to turn the cooling fan effectively limits the rpm. At the highest rpm, you often hear the engine “four stroking” misfiring every other revolution because there is so much exhaust left over from the last power cycle that it takes two reloads to get a fuel/air mixture that’s not too diluted with exhaust from the last power cycle to ignite.

Lots of info, Very informative.

The engine in question is a Honda GX25, Its a 4 stroke, I looked at it last night and used it last night and It doesn’t seem to have a governor, It appears the throttle cable goes straight to the butterfly in the carb. Its a odd 4 stroke in the fact that it can be used in any position, sideways, ect…

I have ran about 7 tanks of fuel thru it and while I was tilling and cultivating away It gives me time to think about such things as fuel consumption at different loads with a presumably fixed throttle.

Its a marvelous little engine, its so relaxing to use the mantis, it calms me.

The two stroke mantis tillers use an echo engine and I believe its the same engine used on some echo weed eaters, I am fairly sure those do not have a governor.

I’ll fold on that one, @Rick. That engine is out of my league. Honda is noted for successfully pushing the envelope in their designs. How difficult is it to start compared to you 2 stroke weed eater?

I’ve been thinking about the Mantis rototiller and whether or not the engine has a governor. I have a small 2 stroke Earthquake tiller. I don’t think it has a governor. However, it has a centrifugal clutch. The tines engage when I speed up the engine and the clutch automatically engages. If the Mantis has the same set-up with a centrifugal clutch, it probably does not have a governor.

I'll fold on that one, @Rick. That engine is out of my league. Honda is noted for successfully pushing the envelope in their designs. How difficult is it to start compared to you 2 stroke weed eater?

It is really easy to start, if you prime it and choke it when cold sometimes it starts on the first pull, if your quick enough to take the choke off it will stay running.

I really like it.

I've been thinking about the Mantis rototiller and whether or not the engine has a governor. I have a small 2 stroke Earthquake tiller. I don't think it has a governor. However, it has a centrifugal clutch. The tines engage when I speed up the engine and the clutch automatically engages. If the Mantis has the same set-up with a centrifugal clutch, it probably does not have a governor.

Yes, the Mantis has a centrifugal clutch. I ran it quite a bit today and im pretty sure theres no governor. That little engine works very hard at times.

How do you like the 2-stroke tiller, @WR? I all always just rent a GX-190 powered tiller once a year and turn over my dad and my gardens. Big honkin’ thing; must weigh 120# or more.

I’d naturally like something easier to use; so I’m asking. (I keep hoping that I’ll find one at the curb; buying small-engine stuff kinda goes against the grain.)

Your dad doesn’t mind being turned over by a power tool ?

Rod Knox said: “I recall some experiments made by bomber pilots in WW II. On the long flights to Japan and back bombers often ran out of fuel and ditched. There was some success in pitching the props to drop manifold pressure to some “magic” level while the rpms were kept at some determined level to reduce fuel consumption . That kind of experimentation may fall in line with your question @Rick. Maybe I can find that information.”

By changing the pitch of the propellers, the pilots were searching for the most optimum setting of speed. Sort like a car with a CVT (Continuously Variable Transmission). What you want is to run the engines at WOT, but at low RPM’s - but not too low. Using vacuum was a good way of finding that point.

@meanjoe75fan–I like the weight and power of the two stroke tiller, but I found it hard to start until I bought the premixed 50:1 ratio that doesn’t contain ethanol. There is a 4 stroke version of this tiller that is sold at Rural King farm stores under the Southland label. The two stroke can be had with an electric starter–I wish I had sprung for that.
As far as tilling is concerned, the Earthquake I have gets the job done. It will bust sod. I like the fact that it is light and easy to lift over the 3 foot fence I have around my garden area.

@meanJoe and @rodknox got the puzzler right. There was no problem with the carburators. The engine RPM was limited by a combination of the V drive gears, the prop diameter, the number # for blades, and propellor pitch. I didn’t query the driver about what RPM he was seeing on the tach so I don’t know how close to maxumum RPM the engine was turning.

@capriracer, I heard that Lindburge worked out a power setting for the B17 bomber which allowed them to be ferried from South America to Africa without a refueling stop. IIRC, the manifold pressure was around the high 20s or up into the low 30’s inches of mercury. You have to remember that these engines were turbocharged AND supercharged. The propellor governors were set down around 1900 RPM. Comon aviation lore is never to never go under square i.e. 25 inches and 2500 RPM or risk detonation. In fact the balked landing ‘go around’ sequence in complex engines is mixture full rich, prop fine pitch (highest RPM) then full throttle (highest manifold pressure).

By juggling airspeed, engine rpm and prop pitch you can find a sweet spot that produces the best fuel mileage and aircraft “range”…In 1940, the jet-stream was still a mystery…By 1944 they had figured it out and learned how to take advantage of it (tailwind) or avoid it (headwind)…Once the jet-stream was understood, aircraft range could be accurately predicted…As Knox said, they also learned how to fine-tune the engines to extend range when necessary…It was the P-51 Mustangs that benefited most from this…

And no this is not spam, but if you want an interesting book talking about squeezing every last drop of fuel out of a decommisioned B-25 WWII bomber, “Spirit of Steamboat” by Craig Johnson. They fly it in a storm on a mercy medical flight with leaky tanks.