Ask the Mechanic: Diesel or petrol powered Kia?

What you need to know:

It will be used to mostly ferry my children to and from school. My dilemma is around whether to purchase a diesel or petrol version

I am looking at the possible purchase of a used Kia Sorento around the 2018-2019 vintage with approximately 70,000 to 80,000kms. It will be used to mostly ferry my children to and from school. My dilemma is around whether to purchase a diesel or petrol version. Please give some guidance.


Diesel fuel is more economical or fuel efficient to run, giving you more kilometres per litre fuelled. This is because diesel has a higher density than petrol, which allows the diesel engine to extract more energy from each litre or volume of fuel.  On the other hand, petrol engine efficiency has been improved overtime to reduce the fuel efficiency gap by new technology such as variable valve timing, electronic fuel injection, stratified or direct fuel injection (D4/FSI/CGI), downsising of gasoline engines and use of continuous variable transmissions (CVT).

The lighter density of petrol engines and spark ignition has for a long time given older petrol engines dominance in the area of instantaneous ignition or quick cold start. However, newer diesel engine technology has given diesel engines faster cold starts and instantaneous ignition, eliminating the need for time wasting manual cold start pre-heating. A newer design of diesel pre-heating glow plugs, which are regulated by the onboard computer and are fitted directly over the combustion chamber rather than the old design pre chamber in the cylinder head; electronically high pressurised fuel common rail which instantaneously delivers the fuel have both improved diesel engine quicker cold start.

Diesel engines have for a long time been better suited for commercial use such as carrying cargo or heavy loads. This is because diesel engines produce higher torque at lower engine revolutions. Petrol engines, on the other hand, produce higher power and torque at higher engine revolutions. This gives petrol engines quicker acceleration and has for a long time made them the preferred choice for smaller passenger cars, where performance and quick acceleration are preferred.  Newer diesel engines have been downsised and twin turbo-charged in conjunction with the other technologies to improve their performance and acceleration, while maintaining their fuel efficiency benefit. This has reduced the performance gap between the two types of engines, making diesel cars strong contenders in the smaller passenger car market segment.

Maintenance cost is another important consideration when choosing between diesel and petrol engines. Diesel engine maintenance (service cost) has for a long time been higher than that of petrol engines. This was especially because diesel engines tended to have bigger engine capacity, requiring more service fluids and oils, as well as the need to replace the diesel filter at each service. Historically, older diesel vehicles were more costly to maintain as they aged. One usually needed to overhaul the fuel system and engines towards 200,000kms or earlier depending on the maintenance standard. This problem was compounded by the development of forced air induction or turbo charging, which put a strain on the older diesel engines.

Newer diesel engines have become less costly as they have been downsised (need less fluids and oils), have longer life fuel filters and are more powerful with use of technology. The introduction of smaller turbo chargers with less boost threshold and twin turbo charging has addressed the need for safer diesel engine performance enhancement with turbo charging.

It should be noted that at higher mileages such as 100,000kms, newer diesel engine fuel system and air intake parts or component replacement tends to be more pricy than petrol engines.

Emission is a subject you cannot ignore when you talk about different fuels. Historically, diesel engines have been known to contribute more to pollution because they emitted more particulate matter and nitrogen oxide gases (Nox). Petrol engines emitted less particulate but also produced polluting carbon monoxide.

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