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How does an LPG Engine work?

19 Apr, 2024 | Business LPG Blogs, Residential LPG Blogs

In this article:

LPG engine workings, benefits, conversion process, precautions, safety best practices, next steps in case of an emergency and more.
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LPG Engines for LPG Vehicles – How Does an LPG Engine Work? How Does LPG Work

When LPG – liquefied petroleum gas – is used in LPG vehicles with internal combustion engines or for stationary engines, like generators, it is called Autogas. Autogas is a varying mixture of propane and butane.
How LPG works in an engine is fundamentally the same as a petrol powered internal combustion engine. The engine block, pistons, spark plugs, ignition system, lubrication system and electricals all work the same on LPG fuel, consisting of propane, butane or a blend of the two and are also referred to as natural gas liquids – NGL.
The two main differences in how LPG works in an LPG vehicles are the fuel itself and the fuel storage and intake systems.
With an octane rating of over 100, LPG works with virtually any petrol engine.
There are 25 million LPG cars & LPG vehicles worldwide.
LPG vehicles (cars) can be OEM single fuel models or dual fuel LPG vehicle conversions that run on either LPG – also known as Autogas – or petrol.

LPG Fuel

LPG – Liquefied Petroleum Gas – is commonly called Autogas when used as fuel for vehicles with internal combustion engines. It can also be used for fixed ICE installations, like generators. Autogas is either propane or a propane and butane mix.

LPG is a fuel gas. Liquefied petroleum gas or liquid petroleum gas – LPG – also identified as propane or butane, are flammable hydrocarbon gas mixtures used as fuel in vehicles, as Autogas, as well as for home heating and cooking fuel.

LPG Systems for Cars – LPG Gas Systems for Cars

There are four main types of LPG gas systems for cars. The newest LPG system for cars – LPDI – is also the most advanced whilst the oldest converter-mixer LPG gas systems are, by far, the least sophisticated car conversions.

LPG Gas Systems for Cars – List

Here are the four LPG systems for cars from newest to oldest…

  • LPG Liquid Phase Direct Injection (LPDI) has liquid LPG injected directly into the combustion chamber. LPDI is the most advanced LPG system for cars.
  • LPG Liquid Phase Injection (LPI) systems inject liquid directly into the intake manifold.
  • LPG Vapour Phase Injection (VPI) the gas exits the converter under pressure and is injected into the intake manifold.
  • LPG Converter-and-Mixer Systems is the oldest style LPG system. Liquid fuel converts into vapour and is then mixed with air before going into the intake manifold.

LPG Car – LPG Vehicle – How Does LPG Work

Autogas is another name for LPG – liquefied petroleum gas – when used as fuel in LPG vehicles with internal combustion engines. It is typically a mixture of propane and/or butane, which can also be sued for fixed applications, like generators.

An LPG car or LPG vehicle is a vehicle that is LPG powered, instead of petrol (gasoline) or diesel. LPG vehicles can be LPG cars straight from the factory, or they can be LPG vehicle conversions.

LPG can fuel various LPG vehicles, including light commercial LPG vehicles, as well as LPG cars.

In different countries, the LPG supplied can be propane, butane or propane-butane blends.

What Does LPG Do to an Engine

In summary of LPG vs petrol engine life, is LPG good for LPG cars and does LPG damage engines:

  • LPG prevents pre-ignition (knocking) in an engine, as it is over 100 octane
  • Manufacturers offer dual-fuel models with the same warranties
  • LPG changes the fuel storage and intake systems of the engine. These are the only major differences
  • LPG conversions to engines do not affect factory warranties
  • LPG doesn’t damage engine parts
  • LPG vs petrol is cleaner burning than petrol or diesel in an engine
  • LPG may actually prolong engine life


What is an LPG Conversion?

An LPG conversion is taking a normal petrol powered vehicle and adding a secondary LPG vehicle fuel system.

Almost all vehicles fuelled by petrol are convertible to LPG vehicle operation at a reasonable cost.

These dual-fuel LPG systems allow an LPG vehicle to operate on either LPG or petrol.

The petrol tank still remains in the LPG vehicle.

The driver can switch from gas to petrol or vice-versa.

The driver is typically provided with a switch and can select which fuel they choose to use.

Having two fuel tanks can double an LPG vehicle’s range.

With both the petrol and LPG tanks filled, dual fuel LPG vehicle cruising ranges of 1000km or more are not unusual.

Advantages of LPG Engines – What Does LPG Do to an Engine – How Does LPG Work

What LPG does to an engine is beneficial and how does LPG work. LPG Autogas engines have lower running costs.

LPG is typically less expensive than petrol or diesel.

Engine oil and spark plugs need changing less often with LPG vehicles, for reduced service costs.

Environmental benefits of LPG vehicles include reduced particulate, CO2 and NOx emissions.

Octane ratings over 100 allows for higher compression ratios, which can increase power output.

Different Fuel Tanks

Did you know that many new petrol cars come with plastic or composite fuel tanks?

Autogas tanks for LPG vehicles are typically made from welded heavy gauge steel.

The steel is generally 3mm to 6mm thick, depending on the tank design.

An LPG vehicle tank design withstands many times its maximum operating pressure.

They are much more puncture resistant and will survive much greater impacts than a typical petrol or diesel tank.

The tank mounting systems are also designed to ensure that the tank will not become dislodged, even in a 20 G impact.

Modern LPG vehicle tanks are compact space saving designs.

The new toroidal LPG tanks are donut shaped and fitted within the spare wheel well.

The spare is replaced by a small emergency inflation kit.

Four Types of LPG Vehicle Intake Systems

There are four types of LPG vehicle fuel intake systems.

Let’s have a look at them from oldest to newest technology:

1. LPG Converter-and-Mixer Systems

Converter-and-mixer systems are the oldest style, dating back decades and still widely used.

The liquid fuel converts into vapour and is then mixed with air before going into the intake manifold.

2. LPG Vapour Phase Injection (VPI)

Vapour phase injection (VPI) systems use a converter-and-mixer system.

The gas exits the converter under pressure and is injected into the intake manifold.

Electrically controlled injectors improve the metering of fuel to the engine, fuel economy and power, and reduce emissions.

This has been the most popular type of LPG vehicle system in recent years.

3. LPG Liquid Phase Injection (LPI)

Liquid phase injection (LPI) systems inject liquid directly into the intake manifold.

That is where it vaporises, not using a converter.

The system works by pumping LPG at high pressure from the LPG vehicle tank to the engine inlet manifold via a set of liquid LPG injectors.

The injectors spray the liquid LPG into the intake manifold.

The fuel vaporising in the intake manifold cools and increases the density of the intake air.

This substantially increases power output, improves fuel economy and has lower emission, when compared to VPI systems.

4. LPG Liquid Phase Direct Injection (LPDI)

Liquid phase direct injection (LPDI) are the most advanced systems.

Liquid LPG is injected directly into the combustion chamber.

The LPG instantly vaporises, cooling the combustion chamber fuel-air mixture during the compression stroke.

This provides further performance and emission improvements.

This cooling effect increases the density of the air-fuel mixture.

So, power and torque are maintained with fewer CO2 emissions.

The effect is like intercooling on turbocharged cars.

An Electronic Control Unit (ECU) controls the various components of the liquid injection system, including the injectors.

LPG injection signal optimises the flow and LPG injection timing.

The ECU is calibrated to the specific vehicle and engine.

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