Economic aspects of LPG-fueled cars


Kazimierz Witaszek, Krzystof Garbala, Mirosław Witaszek, Tomasz Cybulko


Fuel expenses have a significant impact on car operating costs. For cars with spark ignition engines, these costs are often reduced by converting cars to LPG. The use of such fuel not only improves the economic aspects of driving a car, but is also more environmentally friendly because of lowered toxicity of exhaust fumes [1, 2]. In order to enjoy the long-term savings related to gas installation in your car, initial purchase and installation costs should be considered. The basic decision making criterion is economic efficiency, expressed, for example, as the period of time required for the gas installation to “pay for itself”.   Due to fuel price variability [3], as well numerous factors deciding on fuel consumption per unit distance [4] it not easy to determine this period.  Two methods of estimating this period are proposed in this paper. One of them is based on the data known during fueling, while the second one is supplemented with the data gathered by the diagnostic interface OBD2 of ELM327 type and stored in the memory of the Samsung Galaxy 4 Mini smartphone with the Torque Pro application installed. The distance which the car should travel in order for the gas installation costs to be recovered may be calculated using these methods.

Technical specifications for LPG powered vehicles and gas installations

The economic aspects of automotive vehicle use were determined based on the data acquired from the operation of a Renault Thalia  manufactured at the end of 2003.  The car was powered by an 8 valve 55 kW naturally aspirated spark-ignition engine of 1390 cm3 in engine cubic capacity. The gathered data cover the period from the purchase of the vehicle in December 2013 to the middle of 2015. For approximately 1.5 year
the car was fueled with 95 octane petrol. At the end of July 2015, an LPG STAG system was installed. The installation consisted of a toroidal tank of
42 dm3 in capacity, Tomasetto Achille multivalve and ACR02 reducer (Fig. 1).  The main features distinguishing the STAG R02 system are its small dimensions and unique design comprising two aluminum castings and a cover made in the ACtherm system, which prevents  gas from cooling, thus providing good thermal insulation. Due to its unique design, the device heats up very quickly, thus enabling the engine to be very quickly switched from petrol to gas. Therefore, no additional work of the controller, such as temperature correction, is required. The essential advantage of the reducer is its high thermal efficiency and resistance to LPG contaminants. The technical specification for the reducer is presented in Table 1.

Teduktor LPG STAG R02
Fig. 1. LPG STAG R02 reducer [5]

Table 1: Technical specification for the LPG STAG R02 reducer [5]

Material Two aluminum alloy castings and a cover made of a hard and resistant  plastic
Weight 1,56 kg
Dimensions 125 x 122 x 89
Max. inlet pressure 30 bar
Outlet pressure 0.9 ÷ 1.5 bar
Gas inlet diameter M10x1
Gas outlet diameter hose Ø12
Water outlet diameter Ø16
Max. engine power 100 kW/ 136 KM
Approval 67 R – 01 6865


LPG installation was fitted with the ACW01 injector rail.  Such injectors are used for sequential gas injection in spark-ignition engine vehicles. They ensure precise gas dosing into the intake manifold for each engine cylinder individually. All the available AC rails are of  high durability, which has confirmed in long-distance road tests performed in various road and weather conditions for various car brands. The AC injector rail has 2 Ω coils to avoid the overloading of control systems. The coils are fitted with IP67 connection socket. The main component of the rail is the body made of anodized aluminum. Stub pipes are made of brass, while seals are made of rubber compounds compatible with other components [6].


listwa wtryskowa ACW01

Fig. 2.  ACW01 injector rail  [6]


Table 2: Technical specification for the ACW01 injector rail [6]

Nominal working pressure 0,95 ÷ 1,2 bar
Max. working pressure 4,5 bar
Working temperature -20° C do +120° C
Injector opening time ~2,1 ms
Injector closing time ~1,5 ms
Performance range 11 ÷ 29 kW/cylinder
Weight 0,48 kg
Max. gas flow 90 l/min przy p= 1 bar


Sequential gas injection was managed by the STAG Qbox Basic controller (Fig. 3). The controller is designed for 4-cylinder engines with indirect petrol injection. The device is based on a 32-bit microprocessor enabling precise gas dosing, the quick response to variable engine operating conditions and the measurement of petrol injection times. The hardware platform is provided with a number of additional features, for example, the possibility to change the injection sequence, support of the  Bluetooth wireless interface and the ability to read engine rpm based on petrol injection signals. The device design enables new functionalities to be added, and thus its further extension [7].


Sterownik QBox Basic

Fig. 3.  STAG Qbox Basic controller [7]


Author’s own research on a LPG-fueled car

From the purchase of the car, each re-fuelling created the opportunity to gain information on the quantity of the fuel, its price and mileometer reading. The data were recorded by using a cell phone with built-in camera, initially Samsung 5610, then the Samsung Galaxy Mini 4 smartphone. This method of data acquisition was selected due to convenience and cell phone availability. Information from photos was consecutively entered in a spreadsheet, thus enabling further calculations and analyses to be made. Key data, divided into periods before and after the LPG system installation, are presented in Table 3. In the period preceding the LPG system installation, lasting 580 days, the car travelled almost 15,400 km, while consuming nearly 1250 dm3 of petrol. The fuel purchase costs borne by the car owner were approx. 1,600 USD. On 22 July 2015, the LPG system was installed in the tested car, and afterwards data related to re-fuelling were gathered over a period of 85 days. In this period the car travelled over 6,600 km, consuming 605 dm3 of gas and 24.4 dm3 of petrol. The total fuel costs were approx. 315 USD.


Table 3. Operating data for the tested automotive vehicle

  Before  LPG installation After  LPG installation
Data acquisition period 2013-12-19 ... 2015-07-22
(580 dni)
2015-07-22 ... 2015-10-15
(85 dni)
Kilometrage, km 15 377 6 629
Petrol consumption, dm3 1 247,5 24,4
Petrol cost, USD 1 593,21 27,73
LPG consumption, dm3 - 605,0
LPG cost, USD - 287,26
Average petrol consumption dm3/100 km 8,11 0,37
Average LPG consumption, dm3/100 km - 9,13
Fuel cost,  USD/100 km 10,36 4,75
Analysis of the results obtained for the LPG-fueled car

A comparison of fuel consumption in the periods before and after the LPG system installation in the tested car is presented in Figure 4. This shows that after the LPG system installation the total fuel consumption increased by more than 17%, while the share of LPG reached 96%. Due to the significantly lower price of gas, the operating costs per 100 km decreased by more than 55% (Fig. 5). When considering this fact, the return on the investment of purchasing the LPG installation should occur after covering about 11,500 km.

Although this analysis is based on actual operating data, it has some faults, as the two different periods of time were compared when noticeable fuel price changes occurred.  Numerous economic and political factors, which are beyond the control of the users, mean that fuel prices are subject to continuous fluctuations, thus it is impossible to omit such variability in an analysis of gas installation economic efficiency. However, an Internet user may find websites containing databases on fuel prices, exchange rates etc. For the purpose of this paper the database from a website [3] was used.


wykres zużycia paliwa na podstawie danych
Fig. 4. Fuel consumption based on operating data
wykres zużycia paliwa na podstawie danych
Fig. 5. Travel costs based on operating data 



Variations in the average annual prices of 95 octane petrol and LPG within the period when the car with gas system installed was used are shown in Fig. 6.  In addition, the prices for filling the car with fuel are marked. On 22 July 2015 both petrol and LPG tanks were filled completely and every time the tanks were filled up after this, it was until the distributor automatically cut off. Obviously, it was necessary to fill the car with LPG more frequently than with petrol, and less than 25 dm3, that is slightly more than half of the fuel tank capacity in the Reanult Thalia car, was consumed during the whole period under consideration.  

Rys. 6. Zestawienie informacji o cenach paliw w okresie po montażu instalacji LPG

Collation of information on fuel prices after the LPG system installation.


Furthermore, it should be noted that the driving style could also change in the period under examination. The period after the LPG installation was characterized by a higher share of long-distance drives. The distance travelled during one day increased almost threefold.  As the car was equipped with a system recording engine operating parameters, including momentary fuel consumption, it was possible to estimate the LPG installation economic efficiency more accurately. The system was based on the OBD2 diagnostic interface of ELM327 type and the Samsung Galaxy Mini 4 smartphone with
the Torque Pro application installed. Information about each distance travelled was saved in the smartphone memory. It should be noted that the Torque Pro application estimates the fuel consumption based on the petrol injector opening times, and therefore the data refer to the predicted petrol consumption because the OBD interface is unable to gather information of gas injector opening times despite having the LPG system installed in the car. The relationship between the average predicted petrol consumption and the distance travelled is presented in Fig. 4. For better clarity, the abscissa (x-coordinate) is a logarithmic ordinate.  It can be seen from this graph that the vehicle covered  distances from 1km to 300 km. More accurate information on the use of the car is shown in Fig. 5. It is clearly visible that the car was often used to travel distances of about 10 km, although more than half of the overall distance resulted from routes of more than 50 km long, where fuel consumption is significantly lower than those of shorter distances. Based on the data recorded by the Torque Pro application and during filling up the car , the LPG consumption and corresponding petrol consumption were compared (Fig. 8).


Zależność średniego, prognozowanego zużycia benzyny od długości trasy
Rys. 8. Zależność średniego, prognozowanego zużycia benzyny od długości trasy
Udział przejechanych odległości od długości trasy
Rys. 8. Koszty podróży na podstawie eksploatacyjnych danych eksploatacyjnych


This indicates that changes in driving style and the distances which are covered have, to some extent, an impact on petrol consumption, since it was lower by 0.5 dm3/100 km than during the initial analysis (Fig. 4). It means the total fuel consumption by the tested car after the LPG system installation increased by almost 27% instead of 17% as estimated initially.  This factor and a noticeable drop in petrol prices   influenced the travel cost estimation per 100 km (Fig. 10).

Driving the tested LPG-fueled vehicle is still significantly cheaper (by 47%) than conventional petrol fueling. In this case, the return on the LPG installation investment will occur after travelling slightly more than 15,000 km, i.e. about 6.5 months at current car usage.

Changes in fuel prices have a very big impact on the estimation of the economic efficiency of LPG. To illustrate this phenomenon, the data related to fuel prices during the period since the car was purchased have been used. The results of computations are presented in Fig. 11.  It follows from this graph than the most economically favorable conditions for the use of an LPG-fueled car occur during the summer months when the gas price is normally the lowest. A significant decrease in petrol prices at the turn of 2014 could have discouraged some drivers from installing LPG systems in their cars.


Zużycie paliwa na podstawie danych prognozowanych i eksploatacyjnych
Fig. 9. Fuel consumption based on predicted and actual data
Koszty podróży na podstawie danych prognozowanych i eksploatacyjnych
Fig. 10. Travel costs based on predicted and actual data


Zmiany w opłacalności montażu instalacji LPG dla badanego samochodu

Fig. 11.   Changes in the economic efficiency of the LPG installation in the tested car



The research methodology presented in this paper enables the determination of fuel consumption and costs for a conventionally fueled car (petrol) and a LPG-fueled vehicle. These costs were compared with reference to the distance travelled.  It has been found that the fuel consumption in the test car increased by about 27% after the conversion from petrol to LPG. However, fuel costs were reduced by almost half. This results from the low price of gas combined with its relatively large share in fuel consumption (approx. 96%). Variations in fuel prices during a year mean that the cost of the purchase and installation of a gas system is paid off after covering a distance between 13,000km and approx. 24,000km and that may take only several months if the car is used intensively

  4. Witaszek K., Witaszek M.: Wpływ wybranych warunków eksploatacyjnych pojazdu samochodowego na zużycie paliwa. Logistyka 4/2015, płyta nr 2.
  5. Katalog produktów STAG - reduktor R02
  6. Katalog produktów STAG - wtryskiwacze ACW02
  7. Katalog produktów STAG - Sterownik STAG Qbox Basic

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