Weight and per person fuel consumption

Posted on June 2nd, 2007 in Opinion by Julian Edgar

2591_7mg.jpgAny vehicle designed to transport people must move two masses – the mass of the vehicle itself and the mass of the people and/or goods it is carrying.

So a bicycle, the most efficient means of transporting just one person, has (say) a total mass of 100kg, of which 85kg (so 85 per cent) comprises the load being carried. (A skateboard does even better.) A Holden Calais weighs 1700kg to carry about 450kg – or to carry 21 per cent of the total mass. A Peugeot 206 GTi has a mass of 1050kg and can carry probably about 350kg, or about 25 per cent of the total.

But these are theoretical maxima.

What if there’s just the driver in the Calais? Then the mass being carried is just 4.5 per cent of the total vehicle weight! More than 95 per cent of the weight moving along the highway is not the primary load being transported!

Even in the lightweight Peugeot, a single person aboard will still mean that 93 per cent of the mass being moved is not the load.

An Airbus A380 has a maximum take-off weight of 560 tonnes, of which 91 tonnes (17 per cent) is payload. But unlike a car, over its life you can expect it to near that 17 per cent payload rather often.

It all makes sense: if you design a car to take four people and their luggage, there’s lots of space, power, brakes, suspension (and so on) that are simply not required when there’s only one person on board. (A Honda CBR1000RR has a mass of 205kg. Carrying one person, the load comprises 28 per cent of the total mass.)

So most efficient in terms of the mass being carried versus the mass being dragged around is to match the vehicle size and carrying capacity to the actual, most frequent use. Obvious, eh? Looking at cars on the road and their number of occupants, you sure wouldn’t think so.

But there’s another way of crunching the numbers, which can give quite a different perspective. It’s to look at fuel economy, on the basis of litres per 100 kilometres per person.

If it’s carrying four people, the Calais does pretty well – say 12 litres/100 km divided by four, or 3 litres/100 km/person. The Peugeot could also carry four (but less comfortably) and get say 2 litres/100km/person.

A Hyundai Santa Fe diesel – which can carry seven – can blow all these away with a consumption of 1 litre/100km/person – better than my two seater hybrid Honda Insight. One set of data suggests the Honda CBR1000RR gets 6.9 litres/100km, so that’s also its litres/100km/person figure. (Which is pretty bad, but an economy motorcycle would probably halve this.)

According to Wikipedia, pedal cycling energy consumption equates to 0.36 litres/100km so the ‘per person’ figure is better than any other form of transport… even better than walking.

Jet airliners get 3 – 4.8 litres/100km/person. Not mentioned so far are diesel trains, which can be as low in consumption as 1.3 litres/100km/passenger – with a full train of course.

So, much like comparing the total mass of the vehicle with the mass being transported, per person fuel consumption is very much governed by how many people are being carried. Large vehicles, if they are carrying lots of people, do very well. (Oh, here’s another – a full bus at 0.6 litres/100km/person.) But if there’s design space for many, and it’s not being used, then per person consumption is usually high.

And of course not mentioned so far is performance: the aircraft is travelling an awful lot faster than the Calais…. Perhaps the best figure would be litres per 100km per person per km/h?

OK, OK so lots of abstract figures.

But let’s look at the cars we drive, and how we drive them. Clearly, from the above, it makes sense to closely match the design with the use.

But where are the single seater commuter cars? There aren’t any. Where are the two seater cars weighing (say) 500kg? There aren’t any. Where are the four seater cars with a mass of 800kg? Just one or two. Where are the crossovers between pedal cycles and the smallest of useable single seat cars, say with a 100cc petrol engine? There aren’t any.

All these vehicles once existed and were sold in commercial quantities. But not now.

Sound boring? Sound like the sort of cars you sure as hell wouldn’t want to drive? Well, maintaining the same power/weight ratio as the Calais in a 500kg car will need just 57kW. It seems like it can’t be true.

But it is.

One Response to 'Weight and per person fuel consumption'

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  1. Bob Wilson said,

    on October 24th, 2007 at 7:17 pm

    I’ve long suspected that roller-blades to the local bus stop is a good start. If the bus isn’t there, keep rolling to the next stop until the bus approaches. But the other alternative, it to carry a laptop.

    Depending upon one’s work, having a laptop extends the office so the morning and evening commute is an extension of the work day. Until we get self-driving cars, the car commute is interesting but not terribly productive time of the day.

    Bob Wilson

  2. Peter said,

    on November 15th, 2007 at 3:28 pm

    http://www.smh.com.au/articles/2007/11/15/1194766810104.html – seems airliners don’t approach maximum payload weight as often as we’d think.

  3. Atul said,

    on November 17th, 2007 at 8:47 am

    I’ve thought about this as well. Diesel trucks get great fuel economy per pound and in some segments, they carry nearly full loads, (80,000 lbs) for regular on-highway trucks here in the U.S.

    As for fuel economy per person, one must consider that every additional person in your family imposes an environmental consequence higher than that of having only one person in a given vehicle that seat multiple people.