Over the past 15-20 years, diesel engine cars have become vastly more popular than ever before. Before the mid-2000s, diesel engines had a bad reputation for being dirty and noisy, as well as being slow and unengaging to drive. Over the years, this perception has wildly changed, partially thanks to racing-car technology filtering down the chain into everyday road cars from top Motorsport series, such as Endurance Racing, where diesel engines were regularly winning 24-hour races.
In the past 10-15 years, diesel engines have been built with turbochargers, making them cleaner, more economical, faster, quieter, and the engine-of-choice for anyone who covers several-thousands of motorway-miles per year.
More recently though, diesel engines have taken a hit and been “demonised” due mostly to scandals on emissions from certain car makers. These stories have forced most car manufacturers to re-engineer their diesel engines to make them cleaner, and also upgrade their petrol and hybrid technology to make them more desirable than diesels again.
Going a step even further; a vast majority of manufactures at the recent Paris and Geneva Motor Shows brought brand new fully-electric or petrol-hybrid prototypes and future production models; showing yet more push away from diesel and internal-combustion as a whole.
We have covered the benefits of all-electric power before, such as 0% BIK and government grants; so for now, with petrols and diesels more closely matched than ever before, let’s have a look at the pros and cons of each internal combustion option.
Despite recent changes, upgrades, and pushes to make diesel engines “cleaner”, petrol still tops the charts on this front. Diesel engines still emit more nitrogen dioxide, which is an immediate harm to humans. New diesel engines are being fitted with particulate filters to limit the amount of harmful particles, but this has not fully solved the problem.
On the other hand, even modern petrol engines still produce higher CO2 emissions than diesels, making them worse for the environment in the long-term. So, neither engine really comes out on top for harmful gasses & emissions.
Looking at more day-to-day factors which influence the decision to choose one over the other, diesel cars over the past 10-15 years have been vastly superior to petrol when it comes to fuel economy and miles-per-galon/litres-per-100km.
Diesel cars are typically more expensive to buy, but cheaper to run, as petrol is the more expensive fuel to buy (at time of writing, the average cost per litre in Dublin is €1.36 for petrol and €1.32 for diesel). Diesels also typically offer better fuel economy, using less litres per 100km – however, modern petrol engines have become much closer in this regard. Modern diesel engines are also no longer the heavy, steel lumps they once were, meaning handling and overall driving dynamics are no longer a negative setback of diesel cars.
In a video comparing the benefits of petrol and diesel, using 2 top-of-the-range Renault Kadjars, Carwow determined that in order to offset the extra cost of buying a Diesel Kadjar upfront, you would need to drive more than 38,000 miles (61,100km) to break even on fuel costs alone. Based on those figures, if you do less than 20,000km per year, buying the petrol-powered Kadjar is the more cost-effective option over several years, despite petrol being the more expensive fuel.
Another key factor to bear in mind is insurance costs. Typically, petrol engines offer similar power outputs at much lower engine-capacities (see the Mazda 6 example below). Insurance companies nowadays are focusing more and more on engine sizes when giving insurance quotes. Using the Mazda 6 example, a 2.2-Litre Diesel model will be more expensive to insure than the 2.0-Litre Petrol model; despite not offering massively different power or economy figures.
So, if running and long-term fueling costs are roughly similar, what about actual driving ability of petrols vs diesels?
As mentioned, all modern diesels come with turbochargers, and have done for close to 20 years. Typically, this means they are better for motorway cruising and long-distance driving. However, Petrol engines are also being fitted with turbochargers nowadays to meet modern emissions targets, so there’s not as much of a gulf in long-distance ability between petrol & diesel anymore. For example, all models in the Nissan Qashqai range, both petrol and diesel, come with turbocharged engines.
Historically, people would choose a petrol engine for city driving as they are quieter and smoother; and diesels if they do long, motorway miles regularly, as diesels are typically more fuel efficient, smoother at high speed, and have more torque, making overtaking at speed easier and quicker.
Petrol engines have caught up though, with most now offering all the same motorway-cruising benefits, as well as the quiet smoothness at low city speeds.
We can take a real-world example from the current Mazda 6. The top-of-the-range Petrol & Diesel engines are quite close on fuel-consumption. The 2.2 Skyactiv-D Diesel managing between 5.3 and 5.9 Litres per 100km. In comparison, the 2.0 Skyactiv-G Petrol offers between 6.7 and 7 litres per 100km. All the while the petrol engine offers better acceleration, top speed and CO2 emissions, and is also less expensive to buy up front.
Mazda are also set to release their revolutionary new Skyactiv-X petrol engine alongside the all-new Mazda 3 later in 2019, which will all-but close the gaps between petrol and diesel engines. The basic idea behind the Skyactiv-X is to offer diesel-levels of fuel efficiency and torque, in a petrol-powered engine.
As mentioned above, the biggest response from car manufacturers to the emissions scandals, was to put more focus towards their petrol-hybrid and all-electric engines, as viable alternatives to diesel and/or internal combustion as a whole.
Several manufacturers have even gone so far as to drop diesel engines entirely, in favour of hybrid petrol-electric engines which offer similar or better emissions, fuel economy, and driving dynamics as standard diesels.
Others, such as Nissan and Renault have put a heavier focus on all-electric cars. The Nissan LEAF is the best-selling Electric Vehicle of all time, with global sales of over 400,000 units; while the Renault Zoe is also in the top 3 best-selling EVs in Europe in 2018, thanks to it having the highest-range of any affordable EV.
Nissan, in particular, are leading the way on this technology and will release a 60kWh battery option for the Nissan LEAF later in 2019. This larger battery will offer more than 400km of range, putting the LEAF on a par with most small petrol and diesel-powered cars, and making all-electric power a fully viable and affordable alternative to internal combustion.
At the 2019 Geneva Motor Show, Nissan also announced a new concept SUV, the IMQ, featuring a revolutionary "e-Power" engine. The battery is the same tech as in Nissan’s LEAF, but it is mated to a 1.5-litre petrol engine. The engine never drives the wheels, though. That’s the job of an electric motor on each axle, which have combined total outputs of 335bhp and 516lb ft. The petrol engine acts only as a range extender to give more juice. This revolutionary hybrid system will be available on Nissans in the very near future.
Well, it still depends entirely on your personal preferences, but it is definitely worth keeping in mind some of the key points mentioned above.
Diesel is still a cheaper fuel to buy on a day-to-day basis and still offers better fuel-consumption levels (in most models). However, diesels are no longer vastly superior at long-distance cruising, nor are they more cost-effective in the long term.
Modern petrol engines offer a better all-rounder option than every before, by continuing to be the city-driving engine of choice, now with the added bonus of turbocharged-motorway-cruising abilities.
All the while, petrol-hybrids and all-electric-vehicles are becoming more and more mainstream as viable alternatives to both petrol and diesel engines; typically offering far better fuel-consumption levels (or no fuel consumption at all) and running costs, without sacrificing all-rounder driving abilities.