Unlike some other Australian public sector bus undertakings, ACTION is taking a cautious approach to compressed natural gas.

Perth, Adelaide and Sydney already have numbers of CNG-fueled buses in service or on order, while other city operators are still in the ‘try before you buy’ mode.

This is not to say that the adoption of indigenous natural gas is completely ‘on the back burner’ as far as environmentally-conscious undertakings are concerned.

Despite what you may think, ACTION is not as favourably placed as other major government undertakings to receive funds for research and evaluation of new technologies; consequently, its development budget must be keenly honed so that the maximum results can be obtained, without compromise, from minimum expenditure.

This means that ACTION may not have progressed as far towards the espousal of CNG operations as other undertakings.

However, the placing of an order with Mack Trucks Australia for some ‘new generation’ Renault PR100-3 buses (a development of the current PR100-2 model incorporating many features of the not-available in Australia, very advanced Renault R312) With state of the art Austral Denning bodies, provided the impetus for a worthwhile evaluation trial using two vehicles fueled by CNG and two virtually identical ‘controller’ units powered by the same engine but in a normal diesel configuration.

Using ‘high tech’ vehicle management equipment and working under similar traffic and service conditions, the performance of the CNG buses will be checked meticulously against the results achieved by the diesel buses.

The first and sixth deliveries from Mack Trucks were earmarked for CNG operations and their bodywork was modified for this purpose during the construction period under the watchful eyes of the project team from both ACTION and Austral Denning.

The PR100-3 buses are just on 12 metres in length and accommodate 49 seated passengers in Saydair Metro 92 seats plus 26 standers. Wheelbase of the two-door buses is 6120mm.

Roof location
So far in Australia, the gas bottles for dual-fueled and dedicated CNG buses have been placed within the wheelbase under the floor. ACTION has taken a different line by locating the bottles in the roof at the front of the saloon with a slightly lowered ceiling, roof pod and inspection hatch.

ACTION development engineer, Roger Payne said: “Positioning the gas bottles in this location has enabled us to retain the extremely low 640mm flat floor height without any steps or plinths, that is a feature of our PR100-3 design.

“We have also been able to optimise our fully laden mass distribution keeping within the legal axle load limits of six tonnes front, 10 tonnes rear, without any need for special concessions.

“With has capacity of 720 water litres, fully fueled, the CNG buses have almost 1000kg increased tare compared with the diesel buses, ADR59 roll-over compliance has not been compromised and additional reinforcement has been added to some frame pillars to satisfy the Gas Code requirements for mounting strength.

“There is another advantage with a roof mounting location for the gas cylinders. With natural gas being lighter than air, there are no special venting problems in case of any gas leakage.”

The engines in the CNG-fueled Renaults are the conventional turbo charged diesels converted to operate on natural gas.

This requires major changes to the inlet manifolds and cylinder heads, also the fitting of spark plugs to provide ignition. Modifications are made to the pistons to achieve a lower compression ratio

The ‘brain’ of the engine is a ‘black box’ electronic management system, supplied by Perth-based Transcom Gas Technologies Pty Ltd, which also provided earlier generation equipment to Transperth for its CNG trials.

Low temperature
The management system has to cope with some problems which arise through the use of CNG.

According to Transcom, when fully charged, the gas temperature can drop as low as -40 deg. C after passing through the pressure regulators. This causes overfueling and potential engine damage.

The intake manifold design can cause more gas being taken in to some cylinders than others causing detonation and engine damage. The fuel flow must also be adjusted for proper control of the turbo charger.

These and other problems were addressed by Transcom in the design of its Engine Management System (EMS) to ensure optimum performance.

The system measures boost pressure, natural gas temperature and pressure, ambient air temperature, engine speed and throttle position as well as boosting the pressure control valve position.

The EMS microprocessor calculates and makes appropriate adjustments to fuel delivery at the inlets to each cylinder. The ignition spark is also controlled and the input performance ‘updating’ is carried out 50 times per second, according to Transcom.

Fuel is introduced to the cylinders through separate gas injectors and is regulated exactly by the microprocessor.

A totally electronic system controls ignition timing, resulting in a ‘drive-by-wire’ method of providing the driver with all the power and acceleration while fully protecting the engine, Transcom Reports.

Diesel Simulation
The manufacturer claims that the EMS is fully integrated and has been programmed to simulate the relevant diesel engine in terms of performance and governing.

Engine speed and throttle position are its main inputs. These formulate the desired load requirement.

The engine is governed at idle and maximum speed only so load requirements equate to the throttle position between idle and maximum engine speed.

Outputs controlled by the EMS include the manifold valve position, gas flow rate, spark advance and boost pressure control valve movement.

Roger Payne said turbocharging of the engine is necessary to obtain ACTION’s required power level of 178kW (240hp), and accurate fueling control for a CNG turbocharged engine has necessitated the state-of-the-art Transcom EMS as over-fueling can seriously compromise engine life.

The EMS control box is located in a compartment behind the off side rear wheel. It is set up and monitored by a diagnostic lap-top computer.

Fuel management
The CNG/diesel evaluation tests will be carried out using a highly sophisticated fuel management system supplied by Senior Australia

By the use of the transponders and sensing equipment at the depot fulling bay, the system will register the vehicle fleet number, km distance reading and the usage of fuel, coolant and engine oil. This system is also able to record the consumption of gas.

In this way, the performance of the four test units (two CNG, two diesel) will be monitored exactly to the second decimal place. The four buses will be operating in route service under as identical conditions as are operationally achievable.

In addition to exact ‘per km’ reports, the Senior Australia equipment provides vital maintenance and servicing data for all vehicles in the fleet.

For the purposes of this trial, the Natural Gas Company in Canberra has provided a CNG refueling station at ACTION’s Woden depot. This site was selected because of the readily availability of natural gas in an adjacent street.

Because only two buses are involved at present, only a small compression and storage facility is provided. Relatively slow filling takes place overnight through overhead hose installations. Filling takes about five hours and this ensures maximum volumetric efficiency in charging the gas tanks.

There are a number of unique and innovative features about ACTION’s CNG evaluation program such as the roof-mounted gas cylinders, the over night refueling, the Transcom and the Senior fuel monitoring system, which will provide completely accurate comparisons.

ACTION is not rushing into CNG simply on the basis of the experience of other undertakings, although Roger Payne spoke highly of the CNG work being done by STA Adelaide. Instead, like its diesohol trials, it is carefully comparing results under correctly related conditions.

In its initial assessment of compressed natural gas as an alternative fuel, combined with its adoption of the Renault PR100-3 concept, ACTION is further enhancing its reputation as a forward thinking industry leader.

We look forward to seeing the tangible results of this major evaluation program and its influence on ACTION’s fleet replacement program.

Truck & Bus Magazine, January 1994, Page 52 – 54

This page was last updated on 7 April 2021