x Abu Dhabi, UAESaturday 22 July 2017

Honda have changed the gears to boost the fuel economy

Honda engineers believe the new six-speed slushbox was responsible for more than 60 per cent of the 2012's fuel economy improvement.

The 2012 Acura TL has a revised, less prominent nose. Courtesy of Honda
The 2012 Acura TL has a revised, less prominent nose. Courtesy of Honda

It's a subtle bit of subterfuge. Harmless really.

Acura, Honda's upscale luxury brand in North America, makes much of the high-tech wizardry it has added to the TL's engine compartment. The claims were needed. The TL, once the mainstay of Acura's line-up, has fallen upon hard times, sales nearly halved in recent years.

Long-time TL owners who had grown with the four-door sedan as it morphed into an ever-bigger, more powerful hot rod cried enough. They declared the fourth generation's restyling uglier than the proverbial dog's breakfast and the fuel consumption far too profligate for a car that, after all, comes from a company known more for its frugality than its opulence.

So 2012 finds a familiar TL but with a substantially revised nose. I'm not sure what to make of it, but I preferred my TL with the prominent snout.

But, I really didn't spend much time focusing on the TL's styling as I ripped through Acura's press kit. What caught my eye straight away was the claim that the base TL's (the entry-level luxury sedan is available with both 3.5L and 3.7L V6s) motorway fuel economy rating jumped from a rather improvident 7.5L/100km driven to an almost Scroogian 6.8. That's a significant 11 per cent improvement with what, at least outwardly, is essentially the same 24-valve, single overhead camshaft V6 that's propelled the car since 2009.

And Acura's public relations spokespeople were playing it to the hilt. They trotted out all manner of new technologies the company claimed responsible for the dramatic improvement: there's a new coating to the pistons, for instance, to reduce internal friction. The company also adopted a fresh air-intake system and some new precious metal, dual-tipped spark plugs that help with the whole fuel economy improvement.

As impressive as these admittedly ingenious alterations are, they really aren't the big reason that the TL's fuel economy improved so much. The real reason that new TL now bypasses fuel stations like a hybrid is because Honda finally replaced the TL's archaic five-speed automatic transmission with a six-speed.

Indeed, according to Acura's own engineers, the new six-speed slushbox was responsible for more than 60 per cent of the 2012's fuel economy improvement. That's right, despite all the other high-tech tweaks, the real reason the TL is now so frugal is simply because, as auto engineers have known for aeons, putting a taller "motorway" gear in the transmission is still the fastest way to fuel economy improvements.

Indeed, while technologies like direct injection and variable valve timing get trumpeted in advertising and brochures for their fuel-sipping benefits, in truth it is the recent proliferation of multi-ratio transmissions that accounts for much of the improvement in traditional (ie non-hybrid) cars' fuel economy. Where once three- and four-speed automatics were the limit, you can now get as many as eight gears (Audi, Lexus, etc) in one smooth-shifting box.

The most obvious of these new multi-ratio automatics is that more of those gears can be overdrives, "tall" gear ratios that keep the engine spinning slowly while cruising down the motorway. The reason this is effective - and forgive this impromptu appearance of my inner geek - isn't so obvious. To wit: a car travelling down the motorway at 100kph requires about the same power from its engine whether it's spinning at 2,000 or 5,000rpm; the energy required determined simply by aerodynamic drag and rolling resistance. Often, it can take only 20 or 30 horsepower to motivate even a large car on a straight flat road (which makes you wonder why we "need" cars with 500hp).

The difference between the two scenarios, however, is that the engine spinning at 5,000rpm will generate much more internal friction than the engine turning over just 2,000 times per minute. And internal friction is a fuel economy killer. Overcoming said friction takes power; power that is completely wasted because it is not driving the car's wheels. So, the slower the engine is spinning, the less friction it generates and, thus, the better its fuel economy.

There are other advantages to the new automatics as well. More gears means more tightly spaced, or "closer", gear ratios. That's gearhead-speak for keeping the engine closer to its optimum operating rpm (usually, again for those of you wearing your pocket protectors while reading this, the engine's torque peak). Modern automatics also now boast more efficient torque converters that "lock up" - ie, provide a direct connection between engine and wheels, thus further reducing fuel consumption.

Despite the commonly held misconception that the road to optimum fuel economy still requires a manual gearbox, it's not at all uncommon to find the automatic-transmissioned version of a new car boasting superior fuel economy (indeed, the manual version of the upscale TL SH-AWD with the upgraded 3.7L V6 uses more fuel than the automatic version).

The TL is, unfortunately, not available in the UAE, but it serves to show that simply adding more gears to the ages-old automatic transmission is still the quickest route to fuel economy gains.