To start our efficiency journey we take diesel fuel and burn it in an engine, then turn an alternator to create AC power. As it's not practical to split these two and measure the power in the shaft between the engine and alternator, I have taken these two steps together.
The generator I selected was the BetaGen7 from Beta Marine and I set this up on a test bench with a set of cheap electrical heaters to provide a load. When we talk about waste energy, heating my garage was certainly wasting energy!
The fuel supply was fed with a calibrated side pipe and valve, so I could stop the flow of fuel from the tank and measure the flow by timing consumption of a known volume. The scale was set using my wife's conical measures from her collection of histocial pharmaceutical equipment. Used to measure potions years ago, and officially calibrated at the time, they were unlikely to have gone out of calibration unless glass ages.
The load was varied by turning heaters on and off, each had two elements so there were many permutations available. I checked the current indication shown on the generator control panel against a current gauge and an oscilloscope, and confirmed that the two agreed, so I used the generator indication for ease of use during the test.
With the generator running and a known load applied, I closed the fuel valve and timed consumption of a known volume of fuel. After some practice I found it easiest to start timing as the fuel level passed one measure on the pipe, and stop timing 10ml further down. Here are the results:
We expect the efficiency to change with engine temperature, with lower efficiencies just after startup and increased efficiency after the engine had reached its proper operating temperature, which is why the first column shows if the engine was heating up or at its normal running temperature. I can't stand numbers, but I can see pictures, so here is the same data in graphical format.
Apart from the expected improvement in efficiency when the engine had warmed up, we see two things from this. Firstly, the maximum efficiency in converting fuel energy into electrical energy was about 24%. Secondly, the efficiency drops off as the power reduces, with less than 15% efficiency at 1.1kW. Of course, when the engine is running but not outputting any electricity the efficiency is 0%. You might think this an odd thing to mention, but diesel powered narrowboats normally sit in locks with the engine at idle wasting fuel.
On Fuel Consumption
Having mentioned fuel consumption, here are the fuel flow results from the same test. As you can see, the generator uses about 0.7 litres an hour just to keep the engine turning over and with the air cooled alternator acting like a large fan.
Another item here is that the fuel flow never exceeded the manufacturer's specified fuel flow of 2.2 l/h.