How Does Diesel Horsepower Compare To Gasoline?

When you are shopping for a new compact tractor it is easy to get confused about what 15 or 20 horsepower can actually do. You might have had experience with a gasoline riding lawnmower, for example, with that much horsepower and were disappointed in how underpowered it felt. 

Diesel horsepower and gasoline horsepower are not really apples to apples. Yes, they are both measured using an industry standard measurement of horsepower but there is one major difference between the two fuels.... torque.... and diesel engines typically have lots of torque. 

A gasoline engine, for example, might be rated at 20 horsepower but that horsepower only comes at very high up in the RPM range and the power curve is very steep meaning that the horsepower tapers off very quickly as the RPMs drop. 

A diesel engine will typically have a maximum horsepower that kicks in at a much lower RPM and the power curve is much flatter meaning that you will get closer to that full 20hp at much wider range of RPM.

So how does torque help you in a real world situation? Imagine mowing a lawn that went just a little too long between cuts. The grass is a little on the taller side and maybe even a little wet.

The gasoline mower starts up, runs to full throttle and starts into the lawn. Immediately the thick grass pulls the engine down a couple hundred RPMs. Since the gasoline engine has a steep power curve, the engine might only make 15hp at this new, lower RPM. So, the engine, now running with 5 less horsepower struggles even more with the thick grass and loses another couple hundred RPM... where it makes even less horsepower. This downward spiral will continue quickly until you slow down drastically or the engine stalls. 

The diesel engine now starts up, runs full throttle, and starts into the lawn. Immediately the thick grass pulls the engine down a couple hundred RPM but, since its power curve is much flatter, it might only have a couple less horsepower at this new RPM or maybe even the same horsepower. The diesel tractor can continue chugging through the grass without slowing nearly as much as the gasoline tractor- despite having the same exact horsepower.