Mileage, Displacement, Emission; What should be important?

When buying a new car, which is most important?

These are the words of BHPian SS-Traveller.

Kitna Deti Hai, or the common query in Hindi about the real-world fuel efficiency of a car, is one of the first questions most Indian customers ask when setting out to buy a car.

Our obsession with the fuel consumption of our vehicles has to do mainly with saving a few rupees at the fuel pumps – a trend that started with increasing fuel prices during the oil crisis of the 1970s, and persisted through the next few decades as smaller and lighter cars with increasingly more fuel-efficient engines and cleaner technologies (fuel injected petrol engines, common rail diesel engines) appeared in the market.

When designing a car and an engine in the last quarter of the 20th century, the primary focus was on fuel efficiency. To this end, vehicle weight and size were reduced, engines were downsized, and newer engine & combustion technologies were introduced.

But as the oil crisis blew over, a new crisis, that of environmental pollution, appeared over the last 3 decades. Fuel efficiency was not the primary criterion for designing engines any more. The engines with the least emissions, measured in grams per kilometre, were suddenly the best engines.

Common sense indicates that a higher fuel efficiency leads to lower emissions of greenhouse gases (GHGs), especially CO². To this end, engine designers started making mini-engines with progressively smaller displacement, even as governments around the world mandated that smaller displacement engines get bigger tax breaks (Japan with their 550cc (later allowed to increase to 660cc) engines, and India’s 1.2L(P) / 1.5L(D) rule).

But then, apart from fuel efficiency and emissions, a third factor popped up on the horizon – the consideration of occupant safety in the event of crashes. Cars started to grow bigger and heavier, with stronger reinforcements and more complex features that, in simple terms, added more inches and kilograms to newer cars. Japan also altered their kei car rules and reduced incentives for buyers of smaller cars and engines.

Yet, in India today, our engines are getting smaller (albeit more powerful on paper), even as our cars get heavier due to the need for better safety tech. When buying a car, we are led to believe that an engine with a smaller displacement (e.g. 1.0L instead of 1.5L) is more fuel efficient (and we also assume that such smaller engines also have lower emissions). In real-world usage, however, we hear of wildly varying reports of fuel efficiency from engines of smaller displacements. The 1.0L Ecoboost engine that Ford introduced some years ago flopped severely, not only in India but even in the USA, due to reliability issues as well as fuel efficiency – even though it was technologically very advanced.

The little Suzuki Alto K10, or the jellybean Zen, were never known for their fuel efficiency, and the WagonR 1.1L went further per litre than the new versions with 1.0L engine.

The 1.2L petrol engine in the XUV3OO is far less fuel-efficient than its siblings which are both larger and have larger displacement engines.

We are now hearing reports of the 1.0L Hyundai Venue returning single-digit km/l figures in city usage.

It stands to reason (and basic chemistry) that an engine with poor fuel efficiency would also be high on carbon emissions, and fuel consumption and CO² emissions are linearly related (I am not sure whether higher fuel consumption = higher emissions of other GHGs such as NOx / SOx, because of emission reduction devices such as catalytic converters and EGRs).

For a given vehicle weight, a certain power & torque output are required from its engine – which can come from either a large displacement engine in a lower state of tune, or a smaller engine tuned to extract higher bhp and lb-ft per cc. In the latter event, we seem to consistently find that a poorer fuel efficiency is the result – which equates to poorer GHG emissions.

So the big questions that we need answers to, are:

  • If an engine with smaller displacement turns out to be less fuel efficient than a larger engine, why are we insisting on using smaller displacement engines worldwide?
  • Are engines with lower displacement (and higher bhp output per cc) less polluting (and more fuel efficient) than bigger displacement engines under different traffic conditions than what we see in India?
  • In other words, is the Indian urban driving cycle so different from that in Europe or the rest of South-East Asia, that the mileage figures of small hypertuned engines are so poor? Or does it have to do with fuel quality?
  • Apart from higher COx emissions, do smaller engines with poor fuel efficiency conforming to the newest emission laws still have higher emissions of other GHGs?
  • Are we in India (and Japan & Europe) invested too deeply in technologies that help create little engines with big power outputs, and it would be an expensive affair to go back to having bigger engines?
  • When buying a new car, which is most important?
    • the displacement
    • the power (bhp) and torque (Nm) numbers
    • the fuel efficiency on paper (ARAI certified mileage figures)
    • the real-world fuel efficiency as reported by actual users

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