Because Natural Gas is a ‘simple’ yet high octane fuel it produces far fewer emissions than other fuels and combusts efficiently. The efficiency and emissions of the engine vary depending on the combustion and injection methods used.
Stoichiometric combustion is when the chemically exact amount of fuel is added to the air so that when the combustion is completed the chemical formula for the fuel is completed:
CH4 + 2O2 = CO2 + 2 H20 i.e. methane + 2 oxygen = carbon dioxide + 2 water
This offers exceptionally clean combustion and exhaust gases. The downside is that the power output of the engine may be lower and its fuel consumption slightly higher when compared with a diesel engine. Due to the reduced efficiency of low sulfur diesels, this is becoming less of an issue.
The lean burn system employs an air/gas mixture that has more air than the stoichiometric ratio in the combustion cylinder. Looked at the other way, it requires less fuel in the cylinder. This can result in lower fuel consumption compared to stoichiometric combustion with the power output usually maintained by turbocharging.
The carburettor is generally used in stoichiometric engines as it can deliver the right balance of fuel for the air entering the engine. Provided the carburettor is located in reasonably close proximity to the intake of the engine and there are not highly variable load demands, the system works very well.
Single Point Injection
Single point injection is, in essence, an electronically controlled carburettor. The advantage is that the gas is delivered more accurately in accordance with engine demand. Again the injector is still some distance from the inlet (like the carburettor) and so its response to quickly changing conditions is not ideal.
Multi Point Injection
Multi point injection has an injector for each cylinder, so the injectors can be placed in close proximity to the cylinder’s intake port. It also enables fuel to be delivered precisely as required to each individual cylinder (called sequential) and enables more sophisticated technologies such as skip-firing to be used. Skip-firing is when only some of the cylinders are operating (the other cylinders are being skipped). This enables even more efficient use of the fuel at low loads, further lowering fuel consumption and unburnt hydrocarbon emissions.
Carburettors v Injectors
As a point of comparison we can look at the fuel delivery systems used by cars to get an idea of the relative sophistication of the various delivery systems. It should be noted that no system is inappropriate, they each have the benefits and costs.
- Carburettors are no longer used on new cars
- Single point injection is currently only used by low cost cars
- Multi point injection is the system used by most cars today and is the most sophisticated system generally available