Want to be fuel efficient? Drink a fewer small sips of petrol!

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

03 June 2007

Robert Kipngetich Cheruiyot asks; “Assuming that two similar vehicles A and B are moving from point x to y. Vehicle A is moving at a speed of 40km/h while vehicle B is moving at 80km/h. Which one is going to consume more fuel or will both vehicles consume the same amount since both have covered the same distance?”

Engines consume fuel in “sips” – in the same way that humans drink hot tea! The ordinary car engine has four cylinders where the petrol is burned. In every revolution of the engine, each cylinder burns one “sip” of fuel. But they don’t all fire at the same time; they do it one at a time in equal intervals.

Before going on, let me first answer another question from a different reader (Kariuki Kabogo) who wants to know why manufacturers of motor vehicles rate the power of engines in “cc”.

The letters cc stand for “cubic centimetre”. The cc rating is the volume of each one of the engine’s scylinders. The bigger the cylinder, the larger the “sip” of petrol it can take, therefore the bigger a fire it can generate and consequently the more the power it can produce.

Now going back to Robert’s question, the amount of fuel consumed on the journey will depend on two things: the total number of “sips” the engine will take and the volume of petrol in each sip.

The total number of “sips” taken depends on how many revolutions the engine makes during the journey. This in turn is determined by the gear in which the car is driven. If it is maintained in first gear, it will make very many revolutions.

On the other hand, if it is driven in top gear (fifth, in most cars), then it will make very few revolutions during the journey. The question then boils down to whether the car can maintain a speed of 40km/h in top gear. The average car can do that comfortably in gear three or four.

However, any vehicle will easily maintain 80km/h in top gear. Thus I would expect the faster car to consume less fuel in that journey. But if the speed goes above 80km/h, the amount of fuel per “sip” becomes important.

The harder you press the accelerator pedal, the larger the amount of fuel per “sip”. Once the top gear is engaged, the total number of sips taken in the journey remains the same, whether the car is fast or slow.

However, to maintain a high speed, the driver needs to press the accelerator pedal harder thereby drinking larger “sips” of petrol (the car, not the driver!). Therefore, if you want the lowest fuel consumption on a journey, drive in the highest gear possible and press the accelerator as lightly as necessary to maintain that gear. In short, be gentle to your car.