Elgas LPG Gas Blog

LPG Engines - How Does an LPG Engine Work?

An engine powered by LPG is fundamentally the same as a petrol powered internal combustion engine.

The two main differences are the fuel itself and the fuel storage and intake systems.

The engine block, pistons, spark plugs, ignition system, lubrication system and electricals all remain the same.

With an octane rating of over 100, LPG is usable in virtually any petrol engine.

There are 25 million LPG powered cars worldwide.

LPG cars can be OEM single fuel models or dual fuel LPG conversions that run on either LPG – also known as Autogas – or petrol.

What Does LPG Mean in a Car?

Autogas is LPG for vehiclesAn LPG car is a car that is LPG powered, instead of petrol (gasoline) or diesel.

These can be LPG cars straight from the factory, or they can be LPG conversions.

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

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

What is an LPG Conversion?

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

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

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

The petrol tank still remains in the car. 

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 a vehicle's range.

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

Advantages of LPG Engines

LPG conversionLPG 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, for reduced service costs.

Environmental benefits 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 are typically made from welded heavy gauge steel. 

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

An LPG 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.

Toroidal LPG tankModern LPG 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 Intake Systems

There are four types of LPG 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 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 pumps LPG at high pressure from the LPG 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.

How Does an LPG Engine Work?

This inforgraphic, courtesy of The Zebra, shows how a 4-stroke engine cycle works with Autogas or petrol:


How an engine works

Environmental Benefits of LPG Engines

Using LPG creates appreciably less carbon dioxide (CO2) than unleaded petrol.

CO2 is the primary greenhouse gas causing long term climate change.

When examined from a “Well to Wheel” perspective, the CO2 benefit is even greater.

This includes emissions associated with the processing and transportation, as well as use.

Diesel CO2 emissions are 29.2% higher than LPG whilst petrol is 26.8% higher than LPG.

LPG cars also produce 95% less ozone and smog causing NOx than diesel engines.

The dirty black smoke that we see coming from diesel vehicles is fine particulate matter.  

These fine particles may be deeply inhaled into the lung, carrying with them a collection of hazardous compounds.

Experts at the World Health Organisation (WHO) say diesel engine exhaust fumes are carcinogenic.

One of the key environmental advantages of LPG Autogas over diesel, as well as petrol, is the near-absence of particulate matter (PM) emissions.




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The information in this article is derived from various sources and is believed to be correct at the time of publication. However, the information may not be error free and may not be applicable in all circumstances.