Alternative Fuel Systems Inc.

Diesel to Natural Gas Conversion

Many cities and countries today have programs to convert older, polluting diesel transit buses and trucks to run on clean, economical natural gas.  Properly implemented, this is an excellent way to quickly reduce fuel costs, clean up the air and reduce noise with minimum capital costs.  A poorly executed conversion program, however, can lead to higher exhaust emissions, much higher fuel consumption, unacceptable power losses, poor durability and high maintenance costs. 

Diesel to natural gas conversion requires careful engineering on the base engine modifications as well as the control system.  The investment required means that these projects only make sense for larger volume fleets.  Following is a very basic overview of modifications required for a successful conversion:

Compression Ratio:  A typical diesel engine has a compression ratio of between 16 and 18 to 1.  CNG usually works best between 10 and 12; so new or modified pistons are required, with an appropriately shaped combustion chamber to allow proper air-fuel mixing.

Spark Plugs:  Diesels don’t have spark plugs; instead they have diesel fuel injectors.  A diesel conversion replaces the injector with a spark plug and may also require an insert to go through the valve cover – depending on the engine.  Spark plug wear is a common problem, and the high compression ratio and use of gaseous fuel requires higher spark voltage than a petrol car.

Valves:  Natural Gas is a dry fuel so valve seats in a converted engine need to be hardened to prevent abnormal wear.  Older engines need valve guide seals to prevent engine vacuum from drawing oil into the combustion chamber.

Thermal Issues:  Spark ignited engines run hotter than diesels.  Such engines may require upgraded thermal management components, including larger oil coolers, larger radiators, and heat shields around exhaust components.

Catalytic Converter:  A catalyst will generally be required to meet emission regulations.  The exception is lean-burn engines, which, if carefully engineered, can meet certain emissions targets without a converter.

Engine Management System:  Your choice will depend on the exhaust emissions requirements, efficiency targets, durability expectations, technology level of the vehicle and peripheral device control requirements such as cruise control, power take-off, automatic transmissions etc.  AFS produces a range of products or can custom design for these varying requirements.