My 12 horse (7500 watt gen head) is good between 1800 - 2000 with impressive vibration above or below.
I'm looking for better economy... at 1800 RPM a lot of fuel is consumed just to run the engine... this consumption goes up exponentially as RPM increases.. The main reason for the inefficiency at higher RPM is the pumping losses.. at 1200 RPM the pumping losses are minimal... but as you start to get much above this threshold the engine really starts to get less efficient.. some years ago I did a little experiment with a 2 valve diesel.. Although it wasn't the petter the results should be similar... First run the engine with no load at 450 RPM and measure the fuel consumed over a period of time... let's say an hour... then run it at 900 rpm again measuring the fuel... it should burn a little more than twice as much fuel at double the RPM... then run the engine at 1800 RPM... If RPM didn't impact efficiency the engine should burn about 4 times the original amount of fuel... but the reality is the engine will burn about 5-6 times the original amount... The inefficiency really goes up as you get above 2000.. yes the engine makes more power but you pay a hefty price in fuel consumption.. this is part of the reason that higher RPM diesels use 4 valves...
IMO slower is better.. less stress on the parts.. and better economy..
Fatty,
My engine testing coroborates your statement. I also am after maximum fuel economy, but at the same time I am after minimal total hourly fuel consumption (not the same thing). I have resigned myself to the reality that is you want maximum power output per gram of fuel you need to load your diesel up and run it around 75-80% max capability, but if you want to keep a genset running lightly loaded, in the interests of using the least amount of fuel you ought to have a means to slow it down below 1800 RPM for reasons you give and use batteries and inverters to get you back to standard 60 Hz line voltage.
I think you really need to have two different displacement diesel generator plants and that is the direction I have gone. My newly added small one is based on a JD175 engine (353 cc) and will eventually charge my battery bank. Exhaust heat is being captured in a heat exchanger to drive a fuel processor and the engine liquid coolant is available to make domestic hot water and heat the building (errr, more like move the thermometer upwards a miniscule amount.
). This small engine doesn`t make much heat throttled down.
My main plant at the moment is a 1.5 litre, 4 cylinder VW Rabbit diesel engine directly driving a 1800 RPM, 3-phase head. It is used to power my machine shop and heat the building from the radiator and radiated engine/exhaust system heat. When not using my machines I often put a wooden spacer in the throttle linkage to close the rack to 1200 RPM. This consumes far less diesel fuel per hour than light/no load operation at 1800 RPM, (due to the phenomenon you speak of) but the shop lighting on it and the reduced but still present heat output is still very useful.
