The Extra Mile

The Extra Mile

TheExtraMile_IconWe recently came across a little snippet that might spark a few pub debates regarding speed humps and fuel consumption. A car tested at steady speeds gave 58mpg in fourth gear at 30mph, but only 52mpg at a steady 20mph in third gear, which supposedly supports the case of those who insist that speed humps and 20mph limits waste fuel, apart from allegedly wearing tyres unevenly and causing suspension damage. But was that a valid test? Most engines, petrol or diesel, will be doing around 1,200 to 1,800rpm at 20mph in third gear, and virtually the same at 30mph in fourth gear, which actually sounds just about right for good economy. But we think that many moaners just gun it in second and third gears, accelerating hard and then braking between speed humps, and stay in thirsty low gears, ready for a quick getaway from 20mph limits, if they even actually obey them. It’s therefore unsurprising if their fuel economy is hit by negotiating such safety measures in that manner. In 30mph limits with speed humps, a steady 20 to 25mph in third gear seems about the right speed, although you’ll possibly have to slow down a bit more for the more severe ones, and maybe drop into second gear. But there’s just no point in speeding up much between humps and we’re of the firm opinion that those who claim tyre wear and suspension problems simply drive too fast over them. If you do speed up and brake just before every speed hump, then the front suspension is already compressed from the forward weight transfer of braking, and therefore quite likely to be damaged when it hits the hump. And it certainly won’t do your shock absorbers or tyres any good either.

Some readers may be aware of the “extreme” global Shell Eco Marathon economy events that take place annually on each major continent. Unlike our UK MPG marathon, they are for one-off single-seater highly specialised lightweight prototypes and events are run on flat, closed tracks. With a required average speed of 15mph over 10 miles, their unladen weights are usually under 50kg, and their aerodynamic drag is unbelievably low. But the universal laws of physics still apply to them, and it’s interesting to know what technology and strategy works in driving them. Since the early days of the event, a major problem has been the source of suitable power units, with off-the-shelf internal combustion units (particularly diesels) vastly overpowered for the speeds and loads involved, even in the case of 50cc moped and 30cc chainsaw engines. You actually need less than one horsepower to cruise at 15mph in such machines, which means you probably really only need a 5cc engine, and there aren’t many units of that size around!

So what developed from this (more recent EVs and fuel cell power excepted) was the aptly named “burn and coast” driving technique, where fairly strong acceleration in the engine’s high torque band is applied up to around 30mph; the engine is then shut down and the aerodynamic machine, probably running on bicycle tyres, is allowed to coast down to a low speed, when the short “burn” speed boost will be repeated, lap after lap. The machine only uses the engine for quite short spells, but working at close to its maximum efficiency. Well, the road cars of today are vastly overpowered for most legal cruising speeds, and sadly these sort of closed track tactics are near impossible to replicate on the road. But you’ll still see some parallels in what is good economy run technique: get up to your cruising speed quickly through the gears, using the engine’s peak torque speed band; lift off the throttle early and coast up to junctions, hazards, and traffic lights with as little braking as possible. You won’t quite match the near 1,000mpg of the more realistic road-legal “Urban Concept Diesel” 2014 European Eco Marathon class winner, but you’ll be making the best of each gallon, and short spells of good hard work like this will help keep its exhaust and emissions control systems in better working order, too.
Victor Harman

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