Ammonia engine

Driving on Ammonia: Critique of Ammonia Fueled IC Engines

Characteristics of Ammonia

Ammonia has a high autoignition temperature of 651°C so it has a high octane rating. This temperature is just slightly higher than the temperatures typically created by the compression stroke in a typical diesel engine with a compression ratio of between 15:1 and 20:1. This characteristic therefore favors a spark ignition regime.

But ammonia has a relatively low flame speed. Flame speeds are important because at high or even medium revs, the piston can outpace the flame during the power stroke, leaving unburnt fuel in the cylinder to be expelled by the exhaust stroke. This is both polluting and wasteful of fuel. Flame speeds for any given fuel depend on the mix of air to fuel. If the mixture is too lean the flame speed can be so low that energy released by burning cannot match the rate of cooling as the piston decends. If this happens, the power stroke fails and the engine stalls. Ammonia's low flame speed limits how low the fuel concentration can be and thus limits the range of power outputs of the engine and particularly affects the engine's ability to tick over. This characteristic therefore favors a compression ignition regime although it can be mitigated by use of multiple spark plugs to each cylinder - an often impractical proposition if you are converting an existing engine.

Ammonia has a boiling point of -33°C and so unless you are in Moscow in the middle of winter, the fuel can be introduced into the induction manifold where like LPG, it vaporizes without the need of a carburetta. Even in Moscow a little pre heating makes this a very easy way to introduce fuel into the engine. But such a method only suits a spark ignition regime. If you want compression ignition to overcome the limits imposed by the slow flame speed, you could have some fun and games with the injectors. Ammonia has a critical temperature of 132.4°C which means that above this temperature it is a gas no matter what pressure it is under. Since the injector is screwed directly into the cylinder head controlling its temperature and that of any ammonia within it, is difficult to say the least. It will start out cold, warm up to an operating temperature that could be below the critical - but probably not reliably below it. In any event the vapor pressure of ammonia varies dramatically with temperature. This means the most practical method of injecting ammonia into the cylinder is as a gas at all times. But since the pressure in the cylinder at the top of the compression stroke is very high (~100 bar), the ammonia injectors would have to include a pre-heater and be fed using a very high pressure pump. These temperature characteristics of ammonia mitigate strongly in favor of a spark ignition regime.

Converting existing engines to Ammonia

From the characteristics of ammonia it is very easy to build an ammonia buring engine if you are starting from a blank piece of paper. We use spark ignition with multiple plugs and take advatage of the high autoignition temperature with a compression ratio of say 15:1 instead of the 10:1 typically used in a spark ignition (gasoline) engine. And that's it. Job done. You have an ammonia engine.

But for a conversion we have two chices, none of them quite ideal. If you start with a gasoline engine the conversion is essentially identical to an LPG conversion. The difference being that the tank and fuel pipe must not contain metals such as copper or zinc or any of their alloys as ammonia will corrode these. Steel is used instead. The tank must also be able to withstand presures of at least 25 bar because of ammonia's higher vapor pressure. The advance and retard regime will also be different as it will have to take account of the different temperature characteristics of ammonia compared to gasoline or LPG and the different ratios of flame speed against fuel concentrations. This implies a refit of the engine managemnt system (EMS) in many cases and in any event we still end up with limited power range and of unreliable tick over.

Conversion of a diesel is actually better but a bit more radical. Take out the injectors and replace them with spark plugs and the means to fire them, then perform the same LPG style conversion as you would for a gasoline engine. You should not perform conversions on diesels with very high compression ratios but anything up to 18:1 should be safe.

The result is an engine that is more fuel efficient than the converted gasoline engine thanks to the higher compression. It also has fewer of the problems associated with the lower ammonia flame speed. This is because the temperature due to the compression stroke gets the fuel air mixture very close to the autoignition point. This then reqires less energy to actually ignite and the flame speed is enhanced. Diesels converted to ammonia have better output power range but still fall short of being able to tick over on very little fuel.

Turbo/Super Chargers, Intercoolers and Variable Valve Regimes

Turbochargers and superchargers boost an engine's power by compressing the air in the intake manifold so that each stroke processes more air and fuel. It effectively increases the engine capacity. But it also slightly raises the temperature of the air in the intake manifold. This slight rise in temperature translates into a big rise in the temperature at the top of the compression stroke and the peak flame temperature when the fuel is burned. In a spark ignition engine the elevated compression temperature could cause knocking but even in a compression ignition engine, the higher peak flame temperature could cause other damage. Therefore an intercooler is used to mitigate the rise in temperature in the intake manifold. If you have converted a gasoline engine with a compression ratio of 10:1 or less, there is a lot of scope for the engine to accept elevated temperature in the intake manifold. The intercooler can be either bypassed or set to cut in at significantly higher temperature than for gasoline. But if you have converted a diesel with a 15:1 or higher compression ratio, the ammonia air mixture could pre-ignite without intercooling. Some engines use a variable valve regime to close off the intake valve at some point before the intake stroke is complete. This effectively reduces the compression ratio and the power rating of the engine. But it gives a noticable improvement in fuel efficiency. As the expansion ratio is unchanged this is now greater than the compression ratio. The exhaust temperature and pressure is lower so the exhaust contains less energy and so less energy goes to waste. Such engines are commonly heavily supercharged to mitigate the reduced power output per unit capacity and freguently have higher would be compression ratios. Ammonia conversions work exactly the same in such engines as they do in engines lacking this regime.