The Offshore Voyaging Reference Site

Applying Power and Torque

When choosing mechanical gear, especially engines, it’s important to understand power and torque. Many people misuse these terms to sell products, so being informed helps us make better decisions.

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Colin Speedie

Hi Eric

two excellent articles, clearly explained and to the point. Lots of would be boat buyers think that adding a turbo is the answer to everything….

Best wishes


John Harries

Hi Eric,

I learned a huge amount from these two articles, but one thing I’m still confused about.

You write that maximum fuel efficiently is a:

At wide-open throttle is approximately at max torque rpm.

And in part one you write:

Assuming we don’t shift gears, when we put our foot on the accelerator pedal, more fuel and air go into the cylinder which creates higher pressures and increases engine torque.

But in my lay person world I think of throttle setting and RPM as being linked so that, unless over propped, max throttle=max RPM. More throttle = more RPM.

Clearly I’m thinking too simplistically. Perhaps what I should be thinking is maximum throttle at a given RPM, or maximum amount of fuel injected that can be burned efficiently (not lugging) at a given RPM?

Could you please expand a bit on the relationship between throttle setting and RPM and how that relates to your two points above.

Matt Marsh

I’m not Eric but I can chime in:

In a non-turbo diesel engine, there is no throttle. The amount of air in each stroke of the cylinder is always roughly the same regardless of load, and the total rate of air flow through the engine is purely a function of RPM. The lever we call the “throttle” controls the governor that controls the fuel injection metering.

If I advance this lever, more fuel will be injected in each pulse of the injector. Thus, more heat energy is released during each combustion event. Since the cylinder volume and the number of moles of air per cylinder are (approximately) unchanged, then by the ideal gas law P*V=n*R*T a higher temperature must cause a proportionally higher pressure. Higher pressure causes more torque to be applied to the crankshaft. Since the load that’s absorbing this torque (i.e. the propeller) is unchanged, the crankshaft speed must increase.

Increasing the crankshaft speed increases the propeller speed, which increases the torque applied by the propeller on the water and vice-versa. At some new, higher speed, the higher torque of the propeller will balance the higher torque that the engine produces at that governor setting, and RPM will stop increasing.

Many marine diesels have an RPM-sensitive component to the governor. If I advance the throttle to the “2500” position, the governor will increase the fuel volume per pulse until the engine reaches 2500 RPM. If I then unload the engine (eg. by sucking air into the propeller), the reduction in absorbed torque will cause RPM to increase. The governor will then reduce fuel flow to maintain 2500 RPM.

Engine efficiency depends very strongly on the difference between the hottest temperature achieved during combustion (Th) and the temperature at which cold exhaust gas is discharged (Tc).

The diesel will get its best fuel efficiency when the peak temperature in the cylinder is at the highest temperature it can reach during a very quick high-pressure combustion event without leaving unburned or partially-burned fuel behind (high Th, low Tc).

It will get its worst fuel efficiency when it is spinning fast at low load, i.e. when it is pumping a tremendous mass of cold air and burning relatively little fuel per combustion event to produce relatively little torque (low Th, low Tc)

Its fuel efficiency will also be poor when it reaches its maximum possible torque at any given RPM, i.e. when there is so much fuel in each shot that some of this fuel doesn’t encounter enough oxygen to completely burn before the power stroke has ended and the exhaust stroke begins (high Th, high Tc).

Spark-ignition engines maintain a constant air/fuel ratio, and adjust their torque by changing the number of moles of air (n, in the ideal gas law) used per cycle. This, in turn, has an effect on the combustion temperature. Your control lever throttles the airflow at the intake, and then the fuel system adapts in real time to match the actual airflow. The net result, after combustion, is similar to the diesel: higher combustion pressure yields higher torque.

So yes, “throttle” setting and RPM are linked…. they’re linked by the relationship between the heat energy released in each combustion event, the pressure produced in the cylinder by that heat, the torque exerted on the crankshaft by that pressure, and the torque absorbed by the load (i.e. propeller) at that RPM.

John Harries

Hi Eric,

Thanks for the heads up on the graphic. Fixed now.

John Harries

Hi Matt and Eric,

Well, as Rob wrote, that took two cups of tea and three read throughs, but I think I now get it, thanks to both of you.

John Harries

Hi Eric,

Great, that nails it for me. Are you OK if I change the article to read something like:

Instead, we can approximate by knowing that peak efficiency:

  • At no load is at idle rpm.
  • At max torque rpm, assuming we had an oversized, or controllable pitch, prop that fully loaded the engine and that the fuel pump was delivering max fuel for that rpm and not too much (lugging).

I think this will become a reference article going forward and so I want to make it as easy as we can to understand.

John Harries

Hi Eric,

That’s great, thanks. I will change it.