Diesel vs Petrol Engines: What’s the Difference?

If you’re looking for a new car, one major decision is whether to choose a diesel or petrol engine. To make that choice easier, let’s break down the key differences in simple terms, so you can decide which engine is right for you.

1. How They Work

• Diesel Engines don’t need spark plugs. Instead, they compress air until it gets really hot, and when diesel fuel is added, it automatically ignites.
• Petrol Engines use spark plugs to ignite a mix of air and petrol. The explosion from this powers the engine.

Because diesel engines rely on this high compression, they are usually more efficient, meaning you can get more kilometres per litre of fuel.

2. Fuel Efficiency

Diesel engines tend to be more fuel-efficient than petrol engines. This is due to several factors, primarily the higher compression ratio of diesel engines, which allows them to use less fuel for each piston stroke. For example, a petrol engine typically has a compression ratio of around 10:1, meaning for every 10 parts of air, only 1 part of fuel is needed.

In comparison, diesel engines have a higher compression ratio, typically around 18:1, meaning for every 18 parts of air, only 1 part of fuel is needed. This allows you to travel further on a tank of diesel than on a tank of petrol, which is especially beneficial for long-distance driving across New Zealand.

3. Towing and Power

• Diesel Engines generate more torque (pulling power) at low speeds, making them great for towing things like boats, trailers or carrying heavy loads. This is why the majority of utes and trucks are diesel.
• Petrol Engines provide smoother acceleration throughout the rev range usually with more horsepower, making them feel faster and more fun to drive, especially if you’re mostly driving around town.

4. Cost of Fuel

• Diesel is usually cheaper at the pump than petrol, but there’s an extra cost: Road User Charges (RUC). This is a tax you pay based on how many kilometres you drive in a diesel vehicle. So, while diesel might save you at the pump, the RUC can make it more expensive overall, especially if you don’t drive long distances regularly.
• Petrol vehicles don’t have RUC since the tax is included in the petrol price. The price of 91 octane petrol is usually around $1 more per litre than diesel.

5. Maintenance

Diesel engines are built to last longer and handle heavier loads. Traditionally, diesel can be more expensive to maintain and repair. Petrol engines tend to be simpler, so repairs are often cheaper.

For instance, a 3.2l Ford Ranger uses 10l of oil, compared to a standard petrol 3l petrol engine containing around 5l to 6l.

6. Emissions

• Diesel engines can release more harmful pollutants like nitrogen oxides and particulates, though modern diesel cars have systems such as DPF filters to reduce this.
• Petrol engines emit more carbon dioxide (CO2), which contributes to climate change. Modern catalytic converter systems help reduce this.

7. Which One Is Right for You?

• Go for Diesel if you need to tow, travel long distances, or carry heavy loads. It’s more fuel-efficient and better for rugged or practical uses.

• Go for Petrol if you’re mostly driving around town or taking shorter trips, and do not require the extra towing capacity

Conclusion

Both diesel and petrol engines have their pros and cons. Diesel is great for power and efficiency, while petrol is better for everyday driving and lower maintenance costs. Think about what you need your car for, and choose the engine that fits your lifestyle best!