Reciprocating Engines

Reciprocating engines can operate on a wide spectrum of fuels including natural gas, diesel, landfill gas, digester gas, etc. With proper maintenance, large engines can last for 20-30 years while smaller engines (<1 MW) tend to have shorter lifespans.

Reciprocating engines have efficiencies that range from 25% to 45%. In general, diesel engines are more efficient than natural gas engines because they operate at higher compression ratios.

In the future, engine manufacturers are targeting lower fuel consumption and shaft efficiencies up to 50-55% in large engines (>1 MW) by 2010. Efficiencies of natural gas engines, in particular, are expected to improve and approach those of diesel engines.

Uncontrolled NOx emissions from ICEs (especially diesel engines) are the highest among DER technologies. Emission rates for a particular type and size ranges of engines vary from manufacturer to manufacturer. Similarly, emission rates for each type of engine within a manufacturer's product line may vary considerably from the smallest to the largest units in the line. Reasons for these variations include differences in combustion chamber geometry, fuel air mixing patterns, fuel/air ratio, combustion technique (open chamber versus PC), and ignition timing from model to model. Selected NOx and CO emission levels for reciprocating engines are listed in the table below:

Three basic types of post-combustion catalytic control systems for ICEs include:

1. Three-Way Catalyst (TWC) Systems - reduce NOx, CO and unburned hydrocarbons by 90% or more. TWC systems are widely used for automotive applications.
2. Selective Catalytic Reduction (SCR) - SCR is normally used with relatively large (>2 MW) lean-burn reciprocating engines. In SCR, a NOx-reducing agent, such as ammonia is injected into the hot exhaust gas before it passes through a catalytic reactor. The NOx can be reduced by about 80-95%.
3. Oxidation Catalysts - promote the oxidation of CO and unburned hydrocarbons to CO₂ and water. CO conversions of 95% or more are readily achieved.

Other performance-related items for reciprocating engines include:

• Startup times range between 0.5 and 15 minutes
• They have a high tolerance for starts and stops
• Compared with combustion turbines, a lower amount of waste heat can be recovered.
• Engine performance ratings are based on an elevation of 1500 feet above sea level. Deratings of about 2-3% for each additional 1000 feet are common.
• Deratings of 1-2% for every 10°F above the reference temperature (usually 90°F) are common.
• IC engine heads and blocks are rebuilt after about 8,000 hours of operation
• Regular oil and filter changes are required at 700 - 1000 hours of operation