Skip to content
HOME » COLUMNISTS » #294 Diesel Motorcycle: The Missing Spark

#294 Diesel Motorcycle: The Missing Spark

Like the maddeningly elusive brass ring, diesel motorcycles continue to be just out of reach as mass consumer items. There are good reasons why.

The last North American rental car I drove, a Ford Fusion, had one of those digital average gas mileage displays on the dash. For all the time I drove it, the display stuck resolutely at 22.7 mpg.
A month before in the UK, I also rented a car. I’d booked a Peugeot 3008, but when I arrived at the rental desk, the agent said, “Do you have a lot of luggage? If not, I can put you in a BMW for the same price …”
The BMW 118D M-Sport hatchback was a hopelessly impractical vehicle with tiny doors and a miniscule trunk area. But it went like a scalded cat. Under the hood was a 2.0-litre, four cylinder turbo diesel producing 143 hp at 4,000 rpm, which would power it to 130 mph said BMW’s website. For the five days I drove it, the average mileage display kept climbing, and was past 50 mpg when I took the car back. According to BMW’s specification, the M118D has combined urban/highway fuel consumption of 63 mpg!
Allowing for the fact that US gallons are only four-fifths of an imperial one, my basic North American sedan was using more than twice as much fuel as a European sports car. Why?
At least part of the answer lies in the type of fuel—gasoline vs. diesel. So what’s the difference between a conventional gasoline engine and a diesel? In a gasoline engine, fuel and air are introduced into the combustion chamber, compressed by a rising piston to typically 200 psi, and ignited by a sparkplug (spark ignition). In a diesel engine, air alone is compressed to a much higher pressure (as much as 600 psi), raising the temperature of the air to around 550 Celsius. Fuel is injected into the hot air in the combustion chamber at close to TDC, where it ignites (compression ignition). In both cases, the expanding combustion gases act on a piston, converting the heat energy into mechanical energy.
We have Rudolph Diesel, a German refrigerator engineer to thank for the engines that bear his name. Diesel’s goal was to improve the 1876 Otto-cycle spark ignition engine, to which end he filed a patent application for a “…apparatus for converting heat into work” in 1892. Diesel demonstrated a working compression-ignition engine a year later.
Because diesel engines run much higher compression ratios than gasoline engines, they have a higher potential thermal efficiency—as high as 50 per cent, compared with the maximum theoretical efficiency of a gasoline engine at around 40 per cent. So diesels can get more energy from the same amount of fuel. And modern direct injection, common rail turbo diesels have broader powerbands and almost no turbo lag compared with the diesels of even 10 years ago.
Forget your dad’s smoke-belching Detroit-builtV-8 diesel pickup that sounded like a cement mixer full of ball bearings, new diesel engines are lighter, spin up more quickly and have almost completely lost their characteristic rattle. Drive a modern European car and it’s hard to tell whether you have a diesel under the hood or not. As a consequence, diesels now account for more than half of all cars sold in Europe.
Diesel engines have always been a challenge in motorcycles, because their higher compression ratios require heavier engine castings, which equal more weight for the same power as a gasoline engine. In India, where economy often trumps performance, Royal Enfield did sell a diesel bike for a while, the Taurus: but its performance was … underwhelming, and it generated seriously bad vibration. Many Enfield owners in the UK have also ditched the stock gas motor for an electric-start Fuji Robin 400cc diesel, which, though it produces just 8.5 hp, also claims potential 200 mpg (imperial) economy.
Since 1980, motorcycle streamlining guru Clyde Vetter has challenged designers to create fuel-miser motorcycles. Winning the Vetter Challenge in 2012 in the conventional pump-fuel class was Fred Hayes, whose Hayes Diesel used just 0.557 US gallons over an 80-mile course for 143.6 mpg. And it’s Hayes, through his company Hayes Diversified Technologies (HDT), who has supplied the US Marines as well as other NATO forces with a diesel motorcycle.
Why diesel bikes for the military? Many forces, including the USMC, have adopted a single-fuel battleground policy, meaning every piece of military hardware from planes and helicopters to tanks and Humvees must run on JP8/diesel type fuel. So gas-powered motorcycles were out. HDT ‘s M1030B1 motorcycle is based on the ubiquitous Kawasaki KLR650 but uses HDT’s own single-cylinder, mechanical injector diesel engine. The 611cc unit has 90-by-96mm bore and stroke and a 20:1 compression ratio, producing 32 hp at 5,700 rpm (a stock KLR makes around 37 hp at 6,200 rpm). From time to time, HDT has announced its intention to make a civilian version of the M1030B1, but none have yet appeared from the Hesperia, California factory.
But for motorcyclists, there is one huge downside with diesel. Splashed on tarmac, it’s slipperier than a live eel dipped in baby oil. Diesel spills are a significant cause of single-vehicle motorcycle crashes in Europe, and a more widespread use of diesel in Canada could make shiny-side-down excursions more common.
So will a diesel motorcycle ever catch on? Given that their principal attribute is economy, perhaps a run up in fuel prices might create the right market conditions. But by then we’ll all be riding electric bikes …

Keep independent motorcycle journalism alive! If you found this article interesting or useful, please consider sharing.