How green are gas trucks?

The arrival of high horsepower natural gas tractor units from some of the major truck manufacturers, the steady expansion of the UK’s gas refuelling network and a substantial discount on the fuel duty levied on gas now mean more operators are beginning to take a serious look at gas power.

The cheaper fuel means the payback period on a gas truck – even with a higher purchase price – can be as little as three years, well within the five year period the government has pledged to maintain the 50% duty differential on diesel.

So depending on an operator’s annual mileage and access to refuelling stations, natural gas (methane) stacks up financially, and gas vehicles have proven to be just as reliable, driveable and easy to maintain as their diesel equivalents. They can also be quieter, useful particularly for urban and out-of-hours deliveries.

But how green are they? Manufacturers state natural gas produces up to 20% fewer harmful emissions than Euro-6 diesel, claims that have been questioned by some observers. Leaving aside renewable biogas and biodiesel, which in theory are both zero carbon emissions, how does conventional gas stack up against diesel in terms of the key pollutants CO₂, NOx and particulates?

Andy Eastlake (pictured) is MD of the Low Carbon Vehicle Partnership (LowCVP), a “public-private partnership working to accelerate a sustainable shift to lower carbon vehicles and fuels” which has carried out extensive research into the environmental impact of gas trucks and buses.

The chemical formula for methane is CH₄ while for diesel it is C₁₂H₂₄, so for a given energy output methane produces less carbon. “Methane is the simplest hydrocarbon fuel and has the minimum amount of carbon in the fuel compared with regular diesel,” says Eastlake. “If you've got a certain amount of energy, you will get less carbon when you burn methane than you do diesel.

“The challenge is that a spark-ignition gas engine is not as efficient as a compression-ignition diesel engine so the dedicated gas vehicles don't have the same level of efficiency converting the energy in the fuel into energy at the crankshaft.

“The one that's coming on to the road now is high pressure diesel injection or HPDI, the Volvo solution, which is effectively dual-fuel. The principle is that it should provide the benefits of diesel engine efficiency with lower carbon gas fuel. There is very limited data so far on HPDI but it is complex to sort out the after-treatment with both diesel and gas.”

Complex after-treatment

The exhaust after-treatment for diesel Euro-6 trucks is not exactly simple, with most manufacturers combining SCR and EGR with a DPF to achieve the required very low NOx and particulate emissions. But natural gas has its own problems as methane is 30 times worse than CO₂ in terms of global warming, so any unburnt gas escaping down the exhaust has to be removed by a catalyst.

“If you get any methane coming through the engine unburnt, it's quite a difficult molecule to catalyse,” says Eastlake. “A high-temperature catalyst is needed and it's not as simple as it is to oxidise diesel particulates. You need a very sophisticated after-treatment system and that will have been one of the main areas that Volvo would have been working on.”

When comparing CO₂ emissions, the proper measure should be well-to-wheel, the total emissions produced in extracting, transporting and burning the fuel. This is a complicated equation, especially as gas can be used either as a liquid (LNG) cooled to -161degC, or as a gas (CNG) compressed to around 250 bar.

“With diesel, it has to be got out of the ground, refined and transported, so a broad rule of thumb is that for every 100 megajoules you get out of the ground, only 86 or 85 of those are available as energy for the truck,” says Eastlake. “One of the interesting things about gas is that there are lots of different potential power losses. It requires quite a lot of energy to liquefy it, but it can be transported far more efficiently because as a liquid it is much more dense.

“There is also energy consumption in compressing gas so the creative thing to do is tap into the high-pressure gas grid. Around the UK, we've got a very good gas network that's high pressure in certain places and then drops down to low pressure for domestic use. If you're trying to take that low pressure gas and compress it up to 250 bar, quite a lot of energy is required. If you can tap into the high pressure gas main that's an efficient way of running single-stage compressors.”

While a lot of natural gas still arrives in the UK by pipeline from the North Sea, increasing volumes are being shipped in by LNG ships from much further afield. At present there are limited facilities to take LNG direct from the import terminal to road vehicle fuelling stations, so much of the LNG is regasified and put into the gas main network.

“That is bonkers,” says Eastlake. “We don't want to be taking LNG, vaporising it, putting it into the grid, and then either re-liquefying or compressing it. Certainly, you don't want to liquefy it again because that doesn't make it efficient at all.

“But there is an awful lot of liquid diesel and gasoline being shipped around the country in road tankers. The gas and electricity networks are pretty efficient at transmitting energy around the country.”

So a well-to-tank comparison of the carbon intensity of gas and diesel is indeed complicated, but once the fuel is in the vehicle, which is cleaner?

The LowCVP published a report last year ‘Emissions testing of gas-powered commercial vehicles’ summarising the results of tests carried out on behalf of the DfT.

“We tried to do as fair a comparison as we could between the diesel and the gas vehicles,” says Eastlake. “In terms of the greenhouse gases, we found some benefits and some negatives. It's touch and go depending on what you’re looking at, plus or minus 5% to 10%. There’s no reason not to choose gas, but there’s limited reason to choose it because you're not making a huge carbon saving.

“The other key question was around the air quality pollutants [NOx and particulates]. If you go back a generation, to a Euro-5 vehicle running on gas, the gas is a cleaner-burning molecule so it starts out cleaner than diesel. But Euro-6 technology has completely changed the game and has massively cleaned up the exhaust of diesel vehicles. With Euro-6, our position is that there is nothing much to call between gas and diesel in terms of emissions of NOx or particulates.

“They’re both incredibly clean.”

Euro-6 diesel engines using AdBlue SCR after-treatment do however produce small amounts of another greenhouse gas – nitrous oxide (N2O) – but how big a problem this might be needs more research.

“We have seen some N2O emissions from trucks,” says Eastlake. “We estimated it was impacting the overall greenhouse gas by a couple of percent. In the worst case it can be a 10% impact on greenhouse gas emissions.”

AdBlue defeat

The potential flaw with Euro-6 diesel engines is the possibility of unscrupulous operators defeating or bypassing the AdBlue system, which would lead to a significant increase in emissions.

“You can’t do that with gas because it uses a passive three-way catalyst. It's still very possible to make a diesel engine run so the emissions treatment doesn't work,” says Eastlake. “There are all sorts of ways of making them dirty if you so desire.”

But if a Euro-6 diesel exhaust after-treatment system is working as it should, then gas could actually emit more pollutants.

“The diesel DPF is very effective in the removal of particulates,” says Eastlake. “Gas engines arguably produce more emissions of the ultra-fine particulates because they don't have a trap on them. Having said that, when Euro-6 diesel DPFs regenerate, you can get some emissions but hopefully that is quite rare and well controlled.

“Where there is a challenge is extreme applications of diesel engines such as refuse collection vehicles. That is a really tough cycle for diesel vehicles. You're running at about 2mpg and exhaust temperatures are really low - that's one of the areas where gas would work, or better still electrification. But getting the diesel after-treatment to work can be a real struggle.”

Gas vehicles work particularly well in long haul, high load applications, which is why the 6x2 high horsepower tractors coming from IVECO and Volvo are so eagerly awaited in the UK. “That's where the efficiency losses are minimised,” says Eastlake. “At low speeds, it's more of a challenge for a gas engine to perform. In a city bus, the gas engine is actually putting out more CO₂ than the diesel, whereas on long haul, that's when they come into their own. Also, the economics make sense if you're using a lot of fuel because gas is taxed less at the moment.”

The current tax differential on gas has been justified because of the claims for its environmental credentials, and Eastlake warns that the dash for gas could be halted if the government takes a fresh look at the facts on emissions.

“There’s a genuine question being raised and that duty differential is up for review this year,” he says. “We're waiting for the latest evidence because we tested some vehicles probably two years ago and it was a close call between gas and diesel. We've just started doing some testing on the latest generation of gas vehicles from IVECO, Volvo and Scania.

“The higher power models have had another couple of years’ development and we're expecting those to be better again. We’re going to run the tests and do a direct comparison with the latest diesel vehicles because diesel isn’t standing still either.”

Instead of giving fossil-based natural gas a leg up over diesel, Eastlake would rather see more incentives for the take up of biofuels. “We would like to see more differentiation of lower-carbon fuels,” he argues. “At the moment, for the operator, there is no financial benefit in running a biofuel over a fossil fuel. There's nothing encouraging the operators to look at bio-methane.”

As part of this equation, the government also needs to level the playing field between the renewable transport fuel obligation and the renewable heat obligation, as currently it is more financially beneficial for producers of biogas to put it into the grid to heat homes than sell it for transport fuel.

“One of the things that the government needs to get clear in its head is where it wants to use this valuable renewable gas,” says Eastlake. “If you want it in transport, we need to make that more attractive than heating buildings. Arguably, they're both equally good for the environment because they’re just replacing fossil gas.

“The question is: what's going to encourage more renewables to come to the market? Within the ‘Road to zero’ [the government’s low emissions transport strategy], there's a clear upward trajectory for renewable fuel in our transport sector. I think we will struggle to push up the blends of biofuel in retail diesel, but in depot-based heavy-duty operations we could go to some more sophisticated bio-blends.

Big batteries

“I'm certainly not an advocate of putting big batteries in trucks. You'd end up with no payload and a massive amount of embedded carbon in building those batteries. There are discussions about electrifying highways and we will certainly see more electrification of city centre vehicles. But if you're trunking up and down the motorway, a diesel or gas engine is a pretty efficient way of covering that long-haul distance.”

So, if Eastlake was a haulier about to invest in a new fleet of heavy duty tractor units, would he go for gas or stick with diesel?

“I wouldn't want to call it. It's a very interesting question,” he admits. Different fleets have taken different views and neither one is wrong or right, and much depends on access to refuelling sites. The key unknown is whether we're going to protect the current duty differential. That means right now, I'd probably be sitting on my hands and waiting until the budget to decide what I'm going to do.”