Running on gas
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The diesel is comparatively kind to the environment in terms of carbon monoxide, hydrocarbons, and even carbon dioxide emissions. But despite significant progress over the past decade it is still the main culprit when it comes to NOx.
Up in Livingstone British Gas has developed some of the greenest 7.5-tonners on the road by simply replacing much of their diesel fuel with something it has plenty of: natural gas (NG).
As well as being widely available NG makes an excellent vehicle fuel. It contains no lead and almost no sulphur so it produces very low particulates and the lowest amount of carbon dioxide of any of the fossil fuels.
Hydrocarbons levels are very low too, and those which are produced are almost all methane which is not regarded as a pollutant because although it is a greenhouse gas it is non-toxic and does not contribute to the creation of smog.
There's nothing new about NG-fuelled vehicles. Successful experiments in the UK date back to 1933, but until the problem of petrol and diesel emissions became widely appreciated there seemed little reason to develop the infrastructure which would be needed to store and supply a new type of fuel.
There are currently over a million Natural Gas Vehicles.(NGVs) running worldwide, from Italy, to Canada and New Zealand. The technology is well proven and a number of companies can supply conversion kits for petrol-engined vehicles such as the Ford Transit.
What makes the Livingstone project so interesting is that the British Gas boxvans have been converted to dual-fuel operation so they can still run on standard diesel or on a combination of diesel and Compressed Natural Gas (CNG). The gas is fed into the engine's inlet manifold at atmospheric pressure but it is stored in the fuel tank at pressures of up to 200bar (3,0001b/in2).
To cope with this pressure the cylindrical gas tanks are made of 5mm high-tensile steel and are pressure tested to well above their normal working pressure. If pressure rises to abnormal levels—for example if the vehicle catches fire—a fusible plug at the neck of the tank is designed to rupture at 340bar or 212°C. Any gas escaping at this pressure is almost impossible to ignite and, being only 60° as dense as air, the expelled gas will soon dissipate. There is a higher risk once the gas is mixed with air but even here the lower explosive limit is a 4.90% gas content in the air, compared with 1.32% for petrol and 1.20% for diesel.
Data on NGVs worldwide shows that the fusible plug protection works well and actual explosions are extremely rare, partly because CNG has an ignition temperature of over 700°C.This also makes it unworkable in a diesel engine, which cannot produce such a high temperature by compression alone—it either needs a spark, as in a petrol engine, or a proportion of diesel in the charge to raise the temperature and start the combustion process.
In a dual-fuel CNG/diesel vehicle the engine retains its diesel pump so that it can start on diesel. Once running the amount of diesel is limited and the energy shortfall made up by CNG.
At full power a typical energy input is 20% dieseU80% gas. As the power is reduced the amount of diesel remains the same but the supply of gas is reduced so that the proportion of gas falls. Below about 20% engine power, or approximately 1,000rpm, the gas supply is cut off and the engine runs solely on diesel.
In a typical local delivery role such as the Livingstone project, where the vehicles are used to deliver household gas appliances, the
average gas replaces about 60% of the diesel. Engines can run on more than 80% CNG but the injectors rely on the diesel passing through them for lubrication and cooling
Even at 200bar gas has a lower energy density than petrol or diesel, so as well as eating into payloads those heavier fuel tanks also restrict range.
For example, a 70-litre high-pressure tank measures 360x920mm (14x36in), weighs 74kg and holds just 11.6kg of CNG at 200bat This amount of CNG has the same energy as 15.5 litres of diesel fuel and gives the vehicle a theoretical range of say, 100km.
A typical 7.5-tonner has a 120-litre plastic tank. To match that range a CNG-powered truck would need 660kg of tanks and fuel, which implies a payload penalty of well over 500kg. Where NGVs come into their own is for local urban deliveries—where emissions cause most problems– covering 150-250km a shift_ The Livingstone vehicles are fitted with two 90-litre tanks and have a range of over 500km using 60% CNG/40% diesel. This is plenty for vehicles which operate out of a single depot and fuelling point.
The kerbvveight penalty for this system is about 200kg. The Livingstone site is equipped with a "quick-fill" refuelling point which stands alongside the diesel pump and looks very similar. Filling the gas tanks is a simple operation and surprisingly quick.
The filler nozzle locks positively on to a non-return valve in the vehicle's system and once a tap on the nozzle is turned on the gas begins to flow under pressure into the tanks The meter reads in kg rather than litres and the gas flow stops automatically when the supply and vehicle pressures equalise.
A commercial installation such as this costs in the region of £130,000; smaller, cheaper refuelling points are available right down to a domestic compressor/refueller which is the size of a small fridge and is designed to fill up a small van overnight.
A short test drive in a converted Leyland Daf Roadrunner 0812, powered by a 5.9-litre Cummins B Series, left us favourably impressed. Starting up on diesel was as normal and once the engine was running the gas was brought in by operating an on/off switch on the dashboard, As the engine speed reached 1,000rpm the gas began to feed in.
This point was unmistakable as the somewhat harsh combustion note of the Cummins softened noticeably as the slowerburning gas took effect and it stayed that way under fairly hard acceleration.
There was a slight hesitancy at this point which some drivers have found a little disconcerting, such as when pulling out of a side road, but it was no problem. In every other respect, the vehicle drove as normal, pulling strongly right up to the rated speed. As the engine speed dropped back through 1,000rpm so the gas supply was cut off and the fuelling reverted to 100% diesel.
Perhaps the most impressive feature was the exhaust, which stayed smoke free even when pulling hard in the mid range.
There is no doubt that, even at the level of technology demonstrated at Livingstone NGVs could make a useful contribution towards reducing pollution in built-up areas, particularly by reducing NOx levels. Further development of specialised engines running on Liquified Natural Gas (LNG) could provide the amount of energy required to operate heavier urban vehicles such as farestage buses, road sweepers and refuse collectors. This sector represents some 150,000 vehicles in the UK.
Adapting these vehicles to operate on CNG or LNG would certainly improve air quality.
Hard-pressed operators are not about to spend perhaps £3,500 a vehicle—not counting the infrastructure costs—but if the Government is serious about reducing exhaust pollution in our towns and cities CNG vehicles deserve serious consideration.
At the moment, the duty on natural gas is set so that the actual operating cost is close to diesel: an immediate way to increase operator interest would be to lower the duty to at least offset the conversion cost.
El by Gibb Grace