Reciprocating engines can be split into two distinct designs; (1) spark ignition engines and (2) compression ignition engines. There are advantages and disadvantages to both in: fuel type, fuel consumption, power output, maintenance requirements, and air emissions. Spark ignition engines typically use a gaseous fuel such as natural gas or propane. Compression ignition engines typically us a liquid fuel such as Diesel.

Spark Ignition Engines: Lean or Rich Burn

The state of the art technology for spark ignited engines is based on natural gas fuel. These engines are bifurcated into two primary designs; lean burn and rich burn engines. The advantage of a lean burn engine is its greater fuel efficiency due to its inherently lower engine knock tendency and higher compression ratio. Since there is excess air during the combustion process, the CO emission level is also very low. The disadvantage of lean burn technology is the production of NOx. Although, the NOx production is lower in a lean burn engine than in a rich burn engine, it is very difficult and expensive to reduce the NOx level from a lean burn engine with emission after-treatment systems. These systems use reagent-based NOx-reduction catalysts or Selective Catalytic Reduction ("SCR") where ammonia or urea is added to the exhaust system. This requires a separate storage tank with additional operation costs.

Benefits of a lean burn engine compared to a rich burn engine:
   · Greater fuel efficiency
   · Lower emissions
   · Higher power density

Rich burn engines compared to lean burn engines of the same size and configuration have only one advantage, the potential application of a Non Selective Catalytic Reduction ("NSCR") after treatment system. This technology has been commonly used in automotive applications for more many years and is proven to reduce emissions by over 90% (CO, NOx, unburned Hydrocarbons). It is also considered relatively inexpensive compared to other methods.

Compression Ignition Engines: Diesel or Dual Fuel

Compression ignition engines are commonly called Diesel engines after the man who founded the principle of compression ignition, Rudolf Diesel. In 1898, Rudolf Diesel was granted patent #608,845 for the Diesel engine. Since then, the fuel commonly used in the compression ignition engines or Diesel engines has also be named "Diesel fuel".

The compression ignition engine is used in many applications for power generation however, its principal use is for stand-by or emergency power. This is due to the Diesel engines ability to start quickly, pick up a large electrical load and run on stored liquid (Diesel) fuel. The greatest drawback to Diesel engines is the air emissions. It is typically several times worse that a similar spark ignition engine running on Natural gas.

Compression ignition engines can also be fueled by a blend of Diesel fuel with another fuel such as natural gas. These engines are commonly known as Dual Fuel engines.

The advantage of Dual Fuel engines are:
   · Lower air emissions than a regular Diesel engine
   · Power density of a Diesel engine
   · Lower operating costs