Question of the Month: How do I navigate the terminology associated with aftermarket vehicle and engine conversions?

July 19, 2011

Answer: The conversion vocabulary can sometimes be difficult to understand, particularly because the terms are not always used consistently. Below is a glossary of terms related to alternative fuel and advanced vehicle and engine conversions, broken down into categories. Please note that this information is meant to be a general Clean Cities reference guide; other agencies and organizations may define these terms differently.

Aftermarket Conversion

  • Aftermarket conversion – An aftermarket conversion is a vehicle or engine which is modified to allow it to operate using a different fuel or power source than the one for which it was originally designed and certified to use. An aftermarket conversion can also be referred to as a retrofit or an upfit.

Key Players

  • Conversion manufacturer – An entity that produces and sells aftermarket conversion systems. A conversion manufacturer may have technicians that install the conversion system or may work with a qualified system retrofitter (QSR) to complete the installation.
  • QSR – An entity that operates independently of, but works closely with, conversion manufacturers to perform aftermarket conversions. QSRs are also commonly referred to as upfitters or installers.
  • Original equipment manufacturer (OEM) – An entity that produces new vehicles and engines “off the assembly line.” Some OEMs produce vehicles and engines that are prepped for conversions, meaning they include components that are compatible with certain alternative fuels.

Aftermarket Conversion Types

  • Non-engine fuel type – Involves changes to the vehicle components (not including the powertrain) to enable the engine to operate on an alternative fuel, but does not involve significant changes to the vehicle’s OEM-certified engine.
  • Rebuild – Involves modifying the original engine to be powered by a different fuel type.
  • Repower – Involves replacing the existing engine with a new, rebuilt, or remanufactured engine powered by a different fuel type.
  • Powertrain – Involves changes to the vehicle powertrain, and could involve installing an additional electric motor.

Resulting Vehicle Fuel Configuration

  • Dedicated –Vehicles that operate exclusively on an alternative fuel.
  • Bi-fuel and Dual-fuel – Vehicles that can operate on a combination of alternative and conventional fuel. The terms bi-fuel and dual-fuel are often used interchangeably and are defined differently by various agencies and organizations, but may include:
  • Vehicles that operate on a blend of an alternative fuel and a conventional fuel stored in one fuel tank; commonly called flexible-fuel vehicles
  • Vehicles that operate on an alternative fuel or conventional fuel stored in separate tanks, using one fuel at a time.
  • Vehicles that operate on an alternative fuel and use a conventional fuel for ignition assist. These vehicles automatically switch over completely to an alternative after the engine has warmed up.

For additional information about vehicle conversions, please refer to the recently updated Alternative Fuels & Advanced Vehicles Data Center Vehicle Conversions website (

In addition, the National Renewable Energy Laboratory will be publishing A Fleet Manager’s Guide to Understanding Alternative Fuel and Advanced Vehicle Aftermarket Conversions later this summer. More information coming soon!