Hero Diesel Scooter Concept

Turbocharged two-wheel transportation for the people?

Hero RNT TDi concept bike

My first reaction to Hero’s announcement of a possible future diesel scooter called “RNT” is positive. First of all, the thing has the same “right” look that the US Army’s Jeep had in 1941. It looks utilitarian without looking stupid. But before we get carried away, let’s read the text. Hero Motors CEO Pawan Munjal said, “We will have to do much research before we bring it to the commercial arena.”

Two RNT options are also mentioned: 1) a tiny turbocharger to increase power; and 2) a modest electric assist to add about one horsepower through an electric motor in the front wheel.

I did some reading of specifications for small industrial diesel engines, enough to find that their stroke-averaged net combustion pressure (BMEP) is around 70 psi. If we then compute a likely power output for RNT’s 150cc diesel, assuming rpm at the 3600 that is usual for this class of engine, we get three horsepower. That immediately tells us why a tiny turbocharger is mentioned as a possible option, along with the electric assist. It also suggests where Munjal’s “much research” might be applied.

Because of their need for robust construction (the small diesels I reviewed all had 20:1 compression ratios) and a precise, high-pressure fuel-injection pump, diesels are expensive. A recoil-start Hatz diesel engine from Northern Tool lists for $1649.99 and electric start adds another $500.

Why, then, a diesel? Because fuel consumption is typically 30 to 40 percent less than that of a gasoline engine of the same power. That’s why big fuel users, such as the trucking industry, go diesel. The same appeal exists where fuel is scarce or very expensive.

Hero CEO Pawan Munjal poses with the turbocharged diesel RNT concept

Why are diesels so economical? Fuel economy improves as compression ratio is raised, but conventional spark-ignition engines are all done in the range of 10.0 to 12.5 because, above that, they run into destructive detonation. Diesels continue to benefit from higher compression all the way to 16:1. Conventional gasoline engines operated at low-to-mid throttle lose a lot of economy to pumping loss (pulling a partial vacuum on the intake stroke consumes power), but since diesels operate un-throttled, they don’t suffer this loss (diesel load control is accomplished by varying only the amount of fuel injected).

Imagine we can afford the cost of the heat-resistant metals in a tiny turbocharger and that it doubles the average combustion pressure. Also imagine that our research gives us a small diesel injection system that works reliably at 5000 rpm. Now our three horsepower becomes eight.

Because diesels are such good sloggers, even our hypothetical three hp could pull that “Jeep” scooter on low gearing. It just won’t have much speed. Strap on everything a farmer has to take to market, plus a child or two and lunches all around, and I’m sure a further-developed RNT can do the job. Remember that the original Army Jeep had only 60 hp!

Hero RNT TDi concept scooter.

Hero CEO Pawan Munjal poses with the turbocharged diesel RNT concept.

Hero 110cc Dash and 125cc Leap hybrid scooters.

Hero HX250R.