On gas guzzling, CO2, horsepower and ‘green’ ...

Oil IT Journal editor Neil McNaughton allows himself an ‘off topic’ editorial, wondering how come with all the data available in car ads, folks don’t really understand the basic physics of getting energy out of gas and turning it into horsepower. How can a 200HP BMW be ‘green’?

The SPE always entreats its members to proselytize in favor of industry—you can even download an educational kit from www.energy4me.org. Personally, I find it hard to slip the topic into the conversation, let alone set up a PowerPoint show. I’m not sure whether what follows is aligned with the SPE’s general drift, nor even if it is really what people want to hear during the dinner party—which is mostly ‘is oil really running out?’ and ‘Where is the oil price going?, questions that I will refrain from answering right now.

My proselytizing this month is only tangentially about the industry and not at all about information technology so I apologize in advance for this abuse of my editorial position. But when I have a bee in my bonnet I just have to let rip.

A casual remark was at the origin of this investigation. A Houston-based colleague questioned the extent to which driving faster increased fuel consumption. I was surprised by this because I ride a bike and I can assure you that it takes a lot more effort to ride at 30 than 20 mph! There probably is a formula with an exponent or so relating speed to consumption, but that is not quite my concern here. What interests me is how, in the face of an amazing amount of information available in car ads and reviews, can there be such a lack of understanding of the basics.

The matter is not helped by a degree of dissimulation by motor manufacturers world wide on the subject of fuel economy and its corollary, CO2 emissions. Which is my first point. It may have escaped you, it seems to have escaped just about every advertising copywriter, regulator and ecologist in the world, but the carbon in the ‘CO2’ can only come from one place—the carbon in the hydro-carbon, gasoline or diesel that goes into your tank. In other words, fuel efficiency (in miles per gallon or liters per 100km) is equal to a constant times the CO2 produced!

But it gets better. Where does the much vaunted horsepower generated from the engine come from? Unless your car has pedals, it can only come from the same place as the CO2, from the calorific value of the fuel consumed. In other words, the engine’s horsepower at a particular regime is equal to a constant times the fuel used.

To summarize the foregoing, we have the following equation which modesty precludes me from calling ‘McNaughton’s Law*,’ although you are free to do so if you wish.

Horse power = a x Fuel = b x CO2

Now you have to ask, why do you see in car ads three figures, for fuel efficiency, for horsepower and CO2—when one would suffice? Could it be marketing, whereby vendors of performance cars present their vehicles as both powerful and fuel efficient—essentially a contradiction? One such ad that caught my eye is for the BMW 123d, described as ‘the first car with more than 200 HP and under 140 g/km CO2 .’ How does BMW square the circle of low CO2 and heaps of horsepower? Is it because the BMW 123d has achieved a breakthrough in efficiency? I suspect not. The answer lies more in what could be complicity on the part of the regulator or stupidity, probably both. To achieve three numbers, power, fuel and CO2 which are intrinsically related, the regulator allows manufacturers to use different tests. It is obvious that the horsepower number is achieved with the accelerator flat on the floor. The fuel efficiency and CO2 numbers are derived from three standardized driving tests (urban, extra urban and motorway). Where’s the catch? By standardizing on a series of programmed driving cycles, no attempt is made to use the car as intended—i.e. benefitting from its ‘sporty’ nature.

This allows both manufacturers and regulators to appear be ‘green.’ Manufacturers can target fuel efficiency at specific engine regimes and leave the playing field wide open for the usual old gas guzzling technology that we have gotten so used to now that we don’t even notice the absurdity of 200 hp and a 70 mph speed limit or whatever it is in your jurisdiction!

While the relationship between CO2 and liters or gallons of fuel consumed is easy and unequivocal, tying in measurements of power—HP or KW is harder. This is because on the one hand, the petrol engine is rather inefficient—producing a lot of heat—so a lot of the calorific horsepower is wasted. On the other hand manufacturers measure horsepower at an intermediate point in the engine—somewhere around the clutch I believe—although every boy racer wants his HP at the wheels. According to an article in the current issue of MIT Technology Review, some 74% of the energy available from the calories in the gasoline is wasted as heat, leaving 26% for measurement on the test bed. If, as is the case for the 123d, the test bed horsepower is around 200, this implies that there were 555 ‘HP’ going in. From this number and the calorific value of gas, we can figure fuel consumption at max power. Fortunately, BMW also provides us with the vehicle’s top speed—which one presumes is achieved by pressing the accelerator to the floor and working the engine at its maximum power. Going back from top speed through max power (losing 74% of it on the way), we can figure how many oil calories were burned in the process and from there, the fuel burned and C02 produced. My calculations are available on www.oilit.com/links/0810_1. It turns out that CO2 emissions at full power are around 600 gm/km equal to around 36 liters per 100km or if you like, 9 miles per US gallon. Just as a rain check, I Wikipedia’d for data on some real gas guzzlers, Formula 1 cars, which burn around 75 liters per 100km or 3mpg US.

All this profusion and confusion of numbers has served one purpose—to ‘educate,’ especially the US consumer, into the belief of a free lunch—where ‘green’ and ‘power’ can happily co-exist. Motors are run ‘green’ in the test and then the twin turbos kick in for lift off! It would be a good idea if we could standardize on a more telling metric. How about making manufacturers quote fuel consumption (and CO2) at maximum power?

* Not to be confused with the ‘McNaughton Rules’—a good Google if you want a laugh at the expense of my clan.

This article originally appeared in Oil IT Journal 2008 Issue # 10.

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