Question: My wife is an estate agent and mother, and does about six trips of roughly 2 km each day. Stop/start driving is therefore not uncommon. My question is: which engine is more suited to this form of driving: a small petrol or small turbodiesel?
Years ago, I would have said the petrol was better suited, but I would appreciate your comments regarding modern compact turbodiesels (fuel economy is not really significant with such a low total mileage).
CHRIS ELSTON, By email
Answer: In your case, choosing a small petrol engine is valid for the following reasons:
• Diesel engines tend to take longer to reach operating temperature (85-95 degrees Celsius) than petrol engines. Therefore, a petrol engine will have a better chance of avoiding the negative impact of an engine running at low temperatures (excessive wear, fuel-consumption penalties, etc.);
• Turbo technology and reliability (on petrols and diesels) have dramatically improved over the last couple of years. However, it is still not a good idea to stop a turbo engine on a regular basis after it has worked hard, as this shuts off the oil supply to a spinning turbo impeller. Wear will increase and can affect reliability in the long run. Most small petrol engines in budget cars are naturally aspirated;
• After-treatment systems found in modern diesel engines’ exhaust systems (see our article on diesel particulate filters in the November 2011 issue) are especially sensitive to stop/start driving patterns because soot production is increased. Many manufacturers advise their customers to avoid a diesel engine fitted with a diesel particulate filter if they do a lot of stop/start driving;
• Small petrol engines tend to be smoother than small diesels.