This article discusses using LNG (liquefied natural gas) to supplement diesel. It’s a concept that’s far from new – in fact Dr Rudolph Diesel suggested it in his original description of his invention (he recommended methane and accurately forecasted the results).
So unlike many devices purported to improve what engines do, gas injection does not involve the local suspension of one or more laws of nature. Further, it reinforces the point that were any such device or technique to approach that claimed, the associated industries would use it themselves.
The technology described here is far from just experimental. It is being used by engine manufacturers, including Caterpillar and (Westport-converted) Cummins truck engines. The reasons why however are mostly to do with emission reduction and there is a substantial difference between the engine manufacturing industry’s efforts, and what some after-market gas/diesel converter suppliers appear to promote.
Gas/diesel operation is currently being Australian government-evaluated. The year-long trial uses Kenworth T404SAR trucks powered by Cummins’s 15-litre engines incorporating Wesport Innovations’ gas/diesel technology. The trial is being jointly run for the federal government by Mitchell Corporation, Sands Fridge Lines, and Murray Goulburn Cooperative.
There are also many fleets of gas/diesel vehicles running in the USA, particularly in California. There, work is also being done on having diesel engines run primarily on LNG, using diesel primarily to initiate the ignition cycle.
Today’s diesel engines approach 50% efficiency. As William Keese, Chair of the California Energy Commission stated in 2000, an efficient diesel-engined 36 tonne truck [even then] used about 3 km/litre; whereas a petrol-engined truck of one tenth that weight would barely achieve 6 km/litre. Generalising, Keese stated that “diesel engines can nearly double the fuel economy of large pick-up trucks, vans and 4WDs.” As many RV owners will be aware recently made common-rail diesel engines do even better still. Today’s diesel campervans in particular can be surprisingly economical.
The major diesel problem is in meeting increasingly stringent emission standards. Today’s much cleaner (low sulphur) diesel fuel has partially reduced the health hazardous large particulates, but diesels still have excessive greenhouse gas emissions.
There is a further motive. Diesel engines run on a wide range of fuels. These include those synthesised from gas, oil shale, tar-sands, biomass, methane hydrates, and even coal. The gas-to-liquid ‘Fischer-Tropsch’ process can produce high cetane, zero-sulphur fuel from natural gas. The world may well run out of petroleum but there’s any number of alternative sources of fuel for diesels.
China is vitally concerned. Many of that nation’s cities have already banned diesel vehicles from urban areas. Major studies into LPG/diesel are under way at the Automobile Faculty at Chang’an University in Xi’an – mostly into emission reduction and other effects of gas injection.
A 2004 paper from Chang’an reports that, by co-burning LPG, smoke emission is significantly reduced, but that at low loads, fuel consumption, and hydrocarbon and carbon monoxide emissions actually increase. The paper suggests this may be overcome by using gas injection at low loads in only some of the engine’s cylinders, using computer control to optimise the mix.
It should be noted that little engine industry/university research is aimed at improving torque, power or fuel consumption. Rather it is toward retaining existing output and ensuring that diesel engines can be adapted to run on alternative fuels within increasingly stringent emission requirements.
Virtually all independent reports suggest that torque/power tends to be unchanged, except at full load, and that the combined specific fuel consumption is likewise not substantially changed for the better or worse.
There is general agreement that gas injection assists emission reduction, but there are still problems at both low and full engine loads.
The more intrepid truckies have long known that injecting LPG into their diesel-engined trucks would increase their power. It was crude, and it worked, but sometimes proved more than the engine could handle resulting in spectacular and noisy outcomes as the engine blew itself apart. It was the right idea but at the wrong time. It awaited affordable computer technology to make it safe. Then, and inevitably, after-market products evolved. This latter industry capitalises on the basic groundwork, but its emphasis is different: it stresses increasing torque output.
So far, there is little but anecdotal evidence to evaluate the claims. The major claims (or at least reported results) are of increased torque. This is at least partly due to an increase in volumetric efficiency. The injected gas enables less oxygen to be needed for effective combustion and that gas provides the additional fuel that in turn may enable more torque to be produced.
Be aware that claims of improving both torque as well as fuel economy are always suspect. So are claims that ‘extra power is developed, but if not used, less fuel is used instead’. Maybe a little – but it’s not that likely.
There are often hints of fuel saving – but that saving is basically monetary. The total fuel consumed tends not to change so the saving is due solely to the existing substantially lower tax on the 20/30% LNG component.
Where fuel consumption claims are made, the argument is commonly that the fuel mix is more completely burned. But across most of the load range some 98% of the fuel is burned in any recently made diesel anyway. Any attributable gain is limited to that partially burned 2% (at least some must remain unburned or there would be zero emissions). That ‘only 75-80% of diesel fuel is normally burned’ is thus misleading nonsense. Were this to be so, diesel powered vehicles would have emissions like mobile tyre-fires. (The technique may however assist older technology engines to a greater extent).
Pollution reduction is usually mentioned – but rarely emphasised. Presumably it is yet to become a buying inventive. It may also be because meaningful pollution testing costs serious money. There is however ample evidence from research studies that greenhouse gas reductions are substantial excepting at full load – when they may increase. This result is likely to be true also of after-market conversions.
Totally untrue is the often-made claim that the gas acts as a ‘catalyst’. Catalysts, by definition, remain chemically unchanged. The LNG gets burned up. It is a fuel not a catalyst.
Rather than commenting on specific vendor claims I quote from a paper prepared by Chang’an University (Comparative Investigation of Diesel and Mixed LPG/diesel Injection Engines, Proc. IMechE, Journal of Automobile Engineering, Vol. 218, No 5/2004.
It concerns a specific test engine, but the results are typical of most such reports: “tests with LPG showed that engine power reduces slightly, fuel consumption and engine noise have almost no change [but there was a minor improvement above 50% load – CR]; pollutant emissions of smoke, CO and NO2 at full load are improved significantly, but the amount of unburned hydrocarbons increases”.
An Indian university study found generally similar results excepting that efficiency improved by 4% or so above mid-load range. The report noted a ‘drastic reduction’ (40-60%) of NOx reductions across most of the load range – but a 38% increase of NOx at full load.
The most common gas/diesel ratio is somewhere between 70-80% diesel and 20-30% gas. Assuming that no overall reduction of fuel quantity is achieved, monetary savings will be made on the cost of the 20-30% gas content relative to the cost of diesel. At a realistic 50% plus difference there’s a fair saving to be made on that component. I leave readers to do their own sums as to whether conversion is worthwhile financially. It may however be for those seeking more torque from an existing engine. Doing so usually increases engine stresses but it could be argued that in this instance gas injection smoothes combustion.
Potential buyers need to be aware that the primary gas/diesel motives of engine designers are different from those of the after-market industry. This is likely to be reflected in the results obtained from engines specifically designed to run on gas/diesel, and existing engines with after-market gas injection. But, unlike many apparently similar activities, gas/diesel injection does have a sound scientific base. It is not a scam.
(One practical drawback of gas/diesel reported by many users is the need to fill from different bowsers - or from different fuel stations.)
An LNG tank of about 30% the size of the main diesel tank is installed under the vehicle and a feed taken via special reinforced hose to a converter/injection unit in the engine bay. Various transducers (sensors) provide information about engine speed and loading etc used to control the gas feed to the engine. There’s also usually a hose from the engine’s radiator system that keeps the gas from freezing in cold weather. A changeover tap inside the vehicle’s cabin enables the fuel to be manually switched from gas/diesel to diesel alone. Where gas is not available, the engine will run on 100% diesel.
Please note that as with all Collyn Rivers’ articles, the text is copyright Collyn Rivers, Caravan & Motorhome Books, Broome 6725. The actual presentation is copyright CMCA. Please contact both parties if you wish to reproduce the article in any form.
