I've been researching this but haven't tried any of it.
1) You could convert a Listeroid to a normal Nat Gas engine. You need a spark plug, lower compression, a carburettor. Too much work for me although maybe if someone has a kit that is known to work well.
2) There is something called HCCI that involves running Nat Gas but still using compression ignition. Very efficient and low pollution To do this:
a) just inject some NG into the intake. Watch out for over speed conditions. Or use a propane carb at wide open throttle (do not restrict intake flow to a diesel).
b) Evidently you need to pre-heat the intake air to run on 100% NG. Ie, run some pipe along the exhaust, then into the intake. Without this, the compression isn't enough to create enough heat to ignite the NG. Around 320F should be in the ballpark.
c) you can only run somewhere in the 1/3 to 2/3 power range. Below this, it won't ignite (too lean). Above this, you will get something similar to spark knock/detonation from too fast burning.
d) I recommend using diesel to start up, warm up, idle and at 100% power.
e) The Nat Gas air/fuel mixture should be somewhere in the 40-50:1 (by weight) range. 1/3 the "normal" mix.
f) normal diesel 17:1 compression should be ok.
3) You can run on a mix of diesel and NG. Maybe 50/50, depending on load. Still too much diesel for me.
I would be interested in building the controls to automatically switch to 100% NG when the load and intake temp are correct.
It's not clear to me if water injection (a mister in the intake) would slow the NG combustion enough to run at full power.
It's not clear to me if running 100% NG creates a lubrication issue. In general NG engines run very clean and last a long time.
It's not clear to me how to build a generator for battery charging that provides a constant load (say 50%). Normally load goes down as the batteries become charged.
I don't care about waste heat - this is a co-gen situation, so even adding a brake to the drive shaft would be ok to control load.