MOTORING: Turning engine off in traffic does not save fuel

Many people switch off the car engine when in a traffic jam and switch it on when the traffic eases and the vehicles are moving. This is in the name of “saving” fuel.

Do we really save fuel or our vehicles end up consuming more fuel when we switch them on and off during traffic jams?

This question has no solid answer because the number of variables involved are just too many. Some of them are fairly obvious: How long does the traffic jam last? Is it a complete gridlock or is it slow-moving? What kind of technological advancement does the car you are driving possess?

Traffic jams that involve coming to a standstill at intervals that exceed five minutes may call for turning off the engine, but repeat this enough times and you are facing two risks. One is draining the battery, which only charges when you are moving, not when stationary.

Starting an engine, especially anything older than 2015 or thereabouts, draws a lot of current from the accumulator so you can see where frequent cranking will leave you: dead in the water without enough juice to turn your engine over.

The second risk involves ageing the engine faster than usual. Cranking an engine also involves slightly larger amounts of fuel compared with normal driving. In a cold engine, despite the fineness of the injector nozzles, the extra fuel delivered means the intake charge is very “wet”, and petrol is a good solvent especially for oil and oil-based fluids. You know where oil is? In the engine. The same engine where this dense cloud of petrol is being puffed every time you crank it.

At every start-up attempt you wash off the oil film from the cylinder walls with that splash of fuel from the injectors. This too has its own pair of side effects: the oil gets contaminated and degraded faster by mixing and dilution with petrol; and with it washed off the cylinder walls, your pistons have no lubrication to slide up and down the bore with ease. You are killing your engine.

Technological advancement came in the form of edification from the good folks at Jaguar Land Rover during the launch of the XE saloon some seasons ago when life was good and I was happy. I have never had any love for start-stop technology in engines; I actively hate it despite its alleged benefits. One of the things that bothered me most about the start-stop technology* are the two things I’ve mentioned above: draining the battery (combined with the ‘whuawhuawhuawhua’ time-wasting onomatopoeia that typifies an engine being started) and low oil pressure from the constant curtailing of the oil pump operation.

“Stop panicking,” they said.

They then proceeded to launch into a jargon-laden technological tirade that I dare not quote here verbatim for fear of sounding like a textbook, so I’ll simplify it. These New-Age engines come with either secondary batteries or capacitors specifically for this kind of scenario: to prevent rapid drainage of juice from the main accumulator during repeated cranking of the engine and to assist with quick starts, what dwellers of Grogan Road refer to as “one kick”.

And the oil pressure? The lubrication system in these engines has a series of non-return valves along the pipeline that maintain oil pressure even when the engine is off; such that even upon being started, it does not take any time for optimum oil pressure to be achieved. This could be very handy in cold climates — and kind of reminds me of a dry sump oil system which would be another handy way of dealing with this.

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