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What happens during the collision of two LPG-fuelled cars?

05.06.2017

Do LPG-powered car pose bigger threat to driver, passengers and other road users during road accident than petrol or diesel fuelled cars?

We examine it by performing a crash test with cars equipped with autogas installations at the Automotive Industry Institute (PIMOT). The test illustrated one of most common accidents on Polish roads, i.e. impacting the rear of a car.

Both cars were equipped with autogas systems properly installed and approved. All system components had relevant certificates. Car engines were running on LPG during test.

It seems that there is a high risk of autogas system leaking in both vehicles, as both in the front and rear parts of LPG-fuelled cars there are important system components containing gas fuel in liquid phase under high pressure.

There were LPG tanks in the rear of both cars. In the hit car (white Opel Astra tourer) a toroidal tank (630 mm in diameter, 204 mm in height) and capacity of 47 litres  installed in the spare wheel cavity. In the hitting car (red Opel Astra, 3-door model) a cylindrical tank of 270 mm in diameter and 792 mm in length was mounted. The tank capacity was 40 litres.

In the front part of the engine bay (just behind the front-end carrier) in both cars there were gas solenoid valves containing liquid LPG and connected to AC STAG R02 reducers. All high pressure system components were connected with rigid gas copper pipes equipped with appropriate fittings.

Leakage from these components could be the most dangerous event due to the release of large amounts of LPG.

The hitting car (red Opel Astra LPG, 3-door model).

As with other similar road accidents in the car hitting an obstacle, the front part of the vehicle was damaged, including the engine bay, where Autogas system components were installed. They included both low pressure gas components (LPG injector rail, gas pressure and temperature sensor, low pressure part of LPG reducer and flexible hoses connecting these parts), and high pressure components containing liquid LPG (high pressure part of reducer-vaporizer, gas solenoid valve and high pressure rigid gas pipe delivering LPG in liquid phase from the tank).

in the front part of the engine bay, just behind the front end carrier the AC STAG R02 reducer-vaporizer was fitted in red Opel Astra. Along with the reducer the gas solenoid valve was installed on a short rigid conduit nipple. LPG in liquid phase was delivered to the gas solenoid valve directly from the tank located inside the boot. LPG in gas phase flowed out the reducer to the gas pressure and temperature sensor and then through the injector rail to the intake manifolds of individual cylinders.  As a result of accident the engine bay in the red Opel Astra was deformed and the reducer-vaporizer located closest to the front of the vehicle was moved. The high pressure part of the Autogas system was leaking, because the rigid nipple connecting the gas solenoid valve to the reducer-vaporizer was damaged.

But, why no gas cloud was visible around the car ? Liquid LPG released from leaks immediately transits into gas phase increasing its volume 270-fold.

To prevent it from happening the STAG PS02 Plus pressure and temperature sensor manufactured by AC S.A. is installed in the low pressure part of the system (downstream the reducer) to measure these parameters. Based on information from this sensor the LPG controller determines the gas fuel dosage according to engine operating conditions.

This sensor is also responsible for switching the drive unit onto petrol when the LPG tank is almost empty. When gas fuel is used, LPG pressure inside the tank decreases, thus also the pressure drops downstream the LPG reducer in the low pressure part of the system. When the gas pressure sensor detects the pressure value from 0.6 to 0.65 bar, that indicates lack of gas in the tank, the engine is automatically switched to petrol.

Such situation (gas pressure drop in the low pressure part of the system) occurred during the crash test. After unsealing of the high pressure part of the system (damage to the rigid nipple connecting the gas solenoid valve to the reducer), pressure rapidly dropped in the low pressure part.

Pressure loss was detected by the STAG PS02 Plus integrated measuring unit, this sensor acted as if the LPG tank was empty. Low gas pressure signal was sent to the LPG controller that closed solenoid valves in the tank and engine bay (the latter was disconnected from the reducer due to crash) and LPG injectors, switching the drive unit onto petrol.

The pressure and temperature sensor used in each IV generation sequential LPG system has been designed to perform not only control and adjustment functions, but also the role of protection (we often forget it). The crash test results indicate that the sensor fulfils this role very well.

Response time to gas pressure loss for this sensor is very short (0.3 s), thus causing that practically there is no leakage of liquid phase.  Therefore, the collision of LPG-powered cars poses no threat.

However, during the collision of LPG-fuelled cars, besides gas outflow, that did not occur even when the high pressure system part were burst,  inertia of LPG system components, especially of the gas tank that is the most heavy, may also pose a potential threat.

A rapid velocity drop during a collision of cars that suddenly stop from a significant speed on the very short road section) generate very large decelerations that act on the entire vehicle and all its components. As a result large inertia forces are developed and must be withstood by all fittings inside the car. This applies also to gas installation fastenings, especially those of LPG tanks.

There was a cylindrical LPG tank in the red Astra. Its weight along with the multivalve and fuel gas contained inside it was nearly 35 kg. In a crash against a fixed obstacle when the car stops over the length below 1 m, deceleration is around 20g, that is the tank mentioned above (mass of 35 kg) acts on fixing points during the collision as if it weighs 700 kg. Such large loads occurring in crashes set down the  requirements of Regulation No. 67 where the minimum deceleration values to be withstood by tank fastening to the car body.

To meet these requirements the tank was secured with four M10 bolts, Class 8.8 to ensure appropriate fastening strength (20g along the longitudinal axis of the car and 8g in traverse direction, as required in Regulation).  After the crash, the tank remained in a place, and fastening to the car body was not loosened, thus indicating that the tank was secured properly.

Tank fastening is a very important aspect of safety of gas installations in cars, since contrary to all valves protecting the system against gas leakage that are operated automatically, the fastening must be made properly by the fitter. This problem is important enough to be presented in a separate section.

The hit car (white Opel Astra tourer)

It also appeared that the risk of tank damage in the hit car was very high.  All the more so, this was a toroidal tank installed in the spare wheel well, located relatively close to the rear part of the body within the crumple zone. The mass of the tank (including LPG and fittings) was above 40 kg, thus in impact against a fixed obstacle, the load of fixing points corresponds to a mass of 800 kg.

Although the rear part of the car body was deformed,  nothing unusual happened. The spare wheel well was moved forward and upward. However, the tank stayed in a place and its high pressure connections remained untouched.

LPG tanks are the heaviest and also the most resistant components of autogas installations - they undergo no deformation in road incidents. They are built with a large margin of safety as results from EEC UN Regulation No. 67 to which they are certificated.

This has been proved in our test crash. The tank in the white Opel Astra remained deformed and was strong enough to stiffen the entire rear of the car body to reduce its deformation. This stiffening caused that the body crumple zone was moved slightly forward, but not endangering persons who could stay in the car.

The tank fastening was proved to be appropriate. Two M12 bolts, Class 8.8, tightened to an appropriate torque and protected against being pulled out from the car body with appropriately sized washers held the tank in a place despite of large deformation of the boot floor.

The gas line connectors (for filling and supplying LPG to the engine) as well as the multivalve were perfectly protected by the engine design. In toroidal tanks all these components are mounted inside the torus beneath the cover, thus forming a gas-tight multivalve enclosure.

The test indicated that even in extreme situations, when deformation and leaking of autogas system occur, the gas installation poses no threat to persons sitting in an affected car.  Neither crashing the front of a car nor a rear end impact, i.e. practically central hit on the LPG tank does not pose any threat, as automatic protecting devices used in the autogas system shut off LPG supply to avoid gas leakage.

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