Horsepower is a misleading thing. You really need more than a rated horsepower to base a machines ability to do work- especially when comparing gas engines to diesel engines. So when the guy down the street snickers that his Craftsman has twice the power of your Yanmar, here are the facts.

You have to consider how horsepower is rated in order to compare diesel engines to gasoline engines in a typical garden tractor. The horsepower ratings that you see on your tractor and the sticker on the engine of your neighbors lawn tractor are peak horsepower ratings. Gasoline powered engines in garden tractors typically have peak horsepower at about 3000 rpm and horsepower falls off quickly below that. Diesel engines in Yanmar tractors typically have their peak horsepower at about 1000-1500 rpm and horsepower stays up throughout most of the engines rpm range. How does that make a difference? A gasoline powered garden tractor has to be at full throttle to achieve its rated horsepower. When the tractor has a load put on it by your neighbor mowing a thick patch of grass or going up a slight incline, for example, the rpm will fall from, let’s say, around 3000 rpm down to 2500 rpm. At 2500 rpm a gasoline engine will have significantly less horsepower than at 3000 rpm. At 2500 rpm, with much less horsepower, the same load (patch of grass or the hill) will pull the engine down even further until the rpm and horsepower are so low that either the engine stalls or you release the load (by shutting off the mower or pushing in on the clutch)

A diesel engine, however, has a much broader power band. Using the same example, the diesel powered tractor running at full throttle encounters the same load as the garden tractor did. The engine rpm will fall from, let’s say, 2000 RPM to 1500 rpm. At 1500 rpm the diesel engine will have roughly the same power as it did at 2000 RPM and will continue to pull the load. The horsepower and rpm will not continue to fall because the diesel engines power is not as dependant on the engine’s rpm.

"Well at least my Craftsman tractor will last longer"
Well... anything is possible. The numbers, however, tell another story.

First, the diesel engine doesn’t have the parts that usually wear out or give problems. There are no spark plugs, rotors, points, or distributor caps like in the garden tractor. There is no carburetor that is going to gum up and be hard to start after being stored for a long period of time like a typical garden tractor. A diesel engine can be stored for extremely long periods of time (years) and start right up.

Second, The diesel engines in Yanmar tractors are water cooled. This allows the engine to run at a more consistent and cooler temperature witch extends engine life. A typical , properly maintained diesel engine can easily run up to 8,000 to 10,000 hours without major service. Ever seen a garden tractor last that long?

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