What do I have to expect if I choose a LPG car?

@Marcin Bajer

The second post of the series that wants to clarify which fuels can respond more appropriately to the needs of each of us is focused on one of the most widespread fuel at the time, an alternative to the petrol and diesel.

In the last 40 years its diffusion has given origin to an intense planning activity that has let some of our companies become world leaders of the transformation kits.

LPG is a fuel, which at ambient temperature and atmospheric pressure is in a gaseous state. It is liquefied by compression at relatively low pressures between 2 and 8 bar, to reduce its size and make its transport cheaper.

The advantage is to make the mixture density about 250 times its density in the gaseous state, thus reducing the volume for the same mass. This makes it possible to use pressure vessels of relatively limited dimensions.

LPG is suitable for small and medium combustion uses, such as the use of a kitchen or heating, in which it is the only valid alternative to methane.

Furthermore, it is an important application in the automotive sector where, in engines with Otto cycle, it is a very good alternative to petrol, because it has a higher calorific value than methane with the same weight (although the methane causes less pollution), and without the problems of corrosion of the valves and valve-seats caused by methanol and ethanol.

lpg

Photo by R4vi

LPG in the automotive sector

On July 2017, in Italy, the current number of LPG cars in the street should exceed 2.5 million cars out of a total of 37 million.

The LPG has a very extensive distribution network with about 3,984 refuelling points (of which 34 out of service) throughout Italy, at the date of publication of this post.

The application of these systems on the car, both initially as an aftermarket and later as an OEM, especially due to the extensive distribution network, has developed considerably with solutions like methane, which however do not optimize the functionality of the engine Otto Cycle for this fuel.

LPG SYSTEM COMPONENTS

For LPG there are two kinds of systems:

  1. Gaseous injection (the most common)

    The liquid LPG is sent to the pressure regulator where the transition from liquid to gas takes place and then sent to the injectors mounted on the intake manifold near the valves as for petrol.

  2. Liquid injection

    The LPG in the liquid state (and not gaseous as on all other systems), is injected directly into the intake manifold near the valves, as for gasoline, promoting a better fill level and therefore better characteristics of torque and power.

The difference in behaviour between the two types of system is underlined above all at high rpm of the heat engine.

We do not always have a stoichiometric mixture due to the injection of the gaseous LPG in the suction phase and because of its pressure relatively low (so large volumes with very short terms), so that, in these conditions, the result shows reduced performances.

This doesn’t happen in the liquid injection because the volume to be injected is small and the pressure is high.

As follows there is the general scheme of both gaseous and liquid plant with the main components.

schema GPL

  • Toroidal or cylindrical tank
  • Multivalve on the tank
  • Refuelling outlet
  • Control unit
  • Pressure regulator (with solenoid valve)
  • Filters (one in gaseous phase and one for the liquid one)
  • LPG injectors

OEMs have now integrated the control unit for LPG into their wiring, ensuring greater integration but consequently locking the access to data.

How a LPG vehicle OEM should be

Despite the great development of LPG-powered cars, the OEMs have only adapted an LPG system, taken from the aftermarket, and calibrated the Otto Cycle engine for this fuel in order to comply with the emissions for the certificaton.

In this section, l’ll show the characteristics that an optimized car for LPG should have, leaving the user the task to search among the cars offered, the one that most closely matches the ideal version:

  • Minimum LPG autonomy at 500km
  • Otto Cycle engine optimized for LPG (compression ratio around 12, ignition advance, specific timing)
  • Gasoline operation with a 15 litres tank with reduced performance to avoid detonation when the LPG ends (in this case the petrol is considered as an emergency and therefore not subject to emission tests)
  • No penalization of the interior volumes of the passenger compartment and luggage one
GPL

Photo by hsivonen

Strengths of the LPG

  1. LPG is a fuel easily available with low environmental impact and with a high energy and calorific performance

    It is extremely flammable, but it is not toxic

  2. LPG is generally considered to be one of the cleanest energy sources, as it does not pollute soil, water and groundwater

    Thanks to the low sulphur content and a complete combustion, with modest quantities of residues, it contributes to reducing the environmental impact, promoting a better air quality and a reduction in the emission of greenhouse gases

  3. LPG offers a good economy (about 50%) compared to petrol in the car use, and the tank in the car is easier to install than the methane one because the dimensions and weight are far lower.

Problems of the LPG

  1. LPG in the gaseous state has a higher density than the air status and this prevents its spreading into the atmosphere; in case of accidental spills it tends to concentrate itself, stagnating on the ground and in the cavities, causing very dangerous accumulation at risk of fire.

    For this reason, in the past, were prohibited (in Italy) to park the cars with LPG tank underground or indoor (in ships, for example). While the LPG plants of construction after 2001, which comply with the ECE / UN Regulation 67/01, can park up to the basement of the garages, even if connecting with another floor below

  2. LPG tanks, especially those fitted in place of the spare wheel, may be damaged, in case the car suffers a strong crash, thus causing LPG to escape into the gaseous state that remains in the area of the accident with a strong fire hazard.
GPL

Photo by Phillie Casablanca

The problem at point 2 is due both to the mounting position and to the tank manufacturing technology, both for the cylindrical and the toroidal one, as it is composed of pressed and welded shells.

The welding area is always critical as it changes the properties of the material to be assembled due to the strong local heating.
In order to reduce the risk of damage, I designed and built, tested and approved a LPG tank to be installed instead of the petrol one (see pictures).

Serbatoio GPL

Serbatoio GPL

The little humps visible on the right picture are due to an internal pressure test at 100 bar.
This solution was much appreciated by security technicians, but the companies involved didnÕt want cover the investments for the supplies and the higher variable cost, using the component already available on the market.

CONCLUSIONS

POSITIVE ASPECTS

  1. Savings on the LPG vehicle of about 40% compared to the petrol version
  2. Widespread distribution of LPG distributors
  3. exoneration of many traffic blocks
  4. Performance almost identical to the “non-gaseous” version
  5. Ordinary maintenance costs virtually nil (I change two filters every 30,000km)
  6. Replacement of LPG cylinder 10 years from the date of testing

NEGATIVE ASPECTS

  1. The LPG tank is installed in a position, even by OEMs, which is not the best for safety effects, especially the toroidal (the one with the doughnut shape)
  2. Not being mandatory there isnÕt the fire test ON VEHICLE with full LPG tank BUT ONLY ON MELTED COMPONENTS
  3. There are restrictions on parking, transit, and transport by boat

*****

My personal opinion is that a LPG car despite some negative aspects is very interesting and therefore it is right it’s marketed successfully.

*****

  1. What do I have to expect if I choose a CNG car?
  2. What do I have to expect if I choose a LPG car?
  3. What do I have to expect if I choose a hybrid car? (Parallel hybrid)
  4. What do I have to expect if I choose a hybrid car? (Hybrid series)
  5. What do I have to expect if I choose a hybrid car? (Parallel/series)
  6. What do I have to expect if I choose an electric car?
  7. What do I have to expect if I choose a hydrogen-powered car?

Translated by Federica Izzo

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