Thermal Efficiency of a Diesel Engine

Diesels engines are load sensitive type producers. The injection system is tied directly to the governor, when the load is minimal the injectors are only opened just enough to maintain rpm. The same can be said under load, the fuel is increased to provide more power to maintain the set rpm. Hope this helps.

Ok thank you, but shouldn't the thermal efficiency of the engine be rather constant. I mean the friction loss, heat loss and everything else should be the same (almost) for any RPM and load or am I wrong?

Ok thank you, but shouldn't the thermal efficiency of the engine be rather constant. I mean the friction loss, heat loss and everything else should be the same (almost) for any RPM and load or am I wrong?

Don't confuse diesels with gas engines. Gas engines have the property of using fuel this way. The gas engine will burn the same quantity of fuel at the same rpm under idle or load. The injector system in a diesel only allows the fuel required for that rpm. The gas engine has a pre-set mixture.

http://www.simetric.co.uk/si_cc2hp.htm

First of all, your single cylinder engine is not going to be the most efficient design for rotating machinery. For a well-designed engine, the efficiency will be low at low load, improve up to some specific load, then begin decreasing as the load exceeds the design load. When one is working with multi-cylinder configurations, there are many options available to "tweak" the design to meet design criteria. If I am reading your data right, your value for 2.356 kW at 2500 rpm seems unreasonably high- I would check the numbers you used to calculate this value. The first two data points suggest that you can "measure" your load significantly more accurately than the last two data points suggest. The last two data points are within the range of what I would expect from this particular engine design. You do not discuss the ambient conditions under which the tests were conducted- air quality, barometric pressure, humidity can all have an effect. Each data point should be determined with multiple measurements at the same load/rpm point to validate them.

A larger, multi-cylinder diesel engine would give you something on the order of 30% thermal efficiency at the peak of the performance curve (when new). A really big, commercial generation station might be able, with proper tweaking and constant monitoring, be able to reach 32% consistently- maybe, under ideal atmospheric conditions, 35% might be achievable temporarily.

This paper may prove useful/interesting:

http://www.scipub.org/fulltext/ajas/ajas56610-617.pdf

the thermal efficiencies will vary due to design/RPM. Many engines are designed to run at a specific RPM and used for a wide range of applications outside of the optimum and are used as is. cylinder head,combustion chamber and intake manifold size (shape,diameter,length) have a great deal to do with fuel and thermal efficiencies as a large amount of heat is exhaust the more air the more exhaust less air = incomplete combustion but as a rule more air is better in a single cyl. eng. intake runner is important for operating RPM because of the pulsation caused by the opening and closing of the valves ideally you want it to open just as the pressure pulse peaks in a stationary eng. this is easy to adjust in automotive this becomes a little more challenging because of the wide RPM range. I hope I haven't muddied the water to much.