Diesel vs Petrol
Last updated 1/1/2019
The age old diesel vs petrol debate. It’s been going on since dinosaurs wandered the earth. This topic is already covered in many articles throughout the internet. Why should I add to the already bloated commentary? Hopefully this article offers some unique information delivered in an original way. Each section focuses on a particular performance measure and compares the pros and cons of diesel vs petrol.
Business and Cost of Ownership
Diesel engines are more expensive than petrol engines. So in terms of initial expenditure, petrol has the upper hand. You’ll get the same car with probably more power but at a cheaper price when going for petrol. The difference is significant, perhaps in the order of a few thousand dollars.
If you’re using the vehicle regularly for long trips and / or at high load, then you can look to industry to see what costs less. Diesel is used universally in industry, agriculture, mining and power generation, both in Australia and overseas. By universally, I mean petrol practically does not exist within these realms. Whether it’s a haul truck, tractor, harvester, locomotive, generator, bobcat, bulldozer, excavator, on road truck, on road 4WD vehicle or on road transport car, diesel is the fuel of choice. Businesses exist to make profits and business decisions are based on economics. So if you’re wondering which delivers lowest total cost of ownership, I think that question has already been answered. That’s the reason why diesel has been universally adopted in business. Diesel engines last longer, cost less to run, breakdown less and depreciate less. The fact that diesel is ubiquitous in business is a pretty strong argument for it. In terms of cost of ownership, there is no need to go into specifics. Business has done that evaluation already, provided you are doing high mileage and / or high loads.
If a failure does occur, depending on the failure, it can be more expensive to get repaired in a diesel compared to a petrol, particularly if the failure is related to the high pressure fuel system that exists in modern diesel engines. So even though diesel engines last longer and the probability of failure is less, diesels can be expensive to fix. So on average the diesel will cost less but occasionally it will cost more. This is something to take into consideration, especially when purchasing old vehicles.
For short distances and / or light loads, the scale tips in favour of petrol. The small saving in running costs of a diesel is more than offset by the extra expense of purchasing it. The long life of diesel is also jeopardised by frequent short trips. The emission systems on diesel engines are also stressed and prone to early failure when used only for short trips and light loads.
Servicing and Maintenance Costs
There is a perception that diesel is more expensive to service than petrol. Servicing a diesel is basically the same as a petrol. The most significant servicing activity is oil and oil filter change. Diesel oil filters are just your standard canister filter, same as petrol, there is no difference in price. Oil for diesel engines is also pretty similar in price and characteristics as oil for petrol engines. In fact many standard oils are suitable for both. Every other service cost is basically independent on fuel type – brakes, air filter, fuel filter, differential oil, cooling system, battery, greasing points – they’re all pretty much the same. One servicing cost that a petrol engine has that a diesel doesn’t are those related to the ignition system – spark plugs, spark plug leads, ignition coil, etc. So actually diesel is cheaper to service because it has no ignition system. That’s what I’ve found from personal experience.
So where does this perception that diesel is more expensive to service compared to petrol come from? Twenty years ago manufacturers specified 5,000km oil change intervals for their diesels, probably because they produced so much soot. This tradition has carried through today despite the fact that the manufacturer’s service interval is usually 10,000km (same as petrol). Many diesel owners change their oil and oil filter every 5,000km, thus doubling their servicing costs. Some people replace their fuel filter at ridiculously small intervals, believing they are protecting their diesel engine. For some reason they think the manufacturer is under-specifying servicing requirements. Or they believe servicing more often will yield a longer operating life. Even if a manufacturer did have a philosophy of under-specifying, I’m not sure why they’d target only their diesel engines. In other words, the arguments to service diesels more frequently than specified are just as applicable to petrol engines. Check out this article for a discussion on service intervals.
I service my diesel hilux exactly according to the manufacturer’s specification, no different to how any fleet owner would operate. In my opinion, having owned both petrol and diesel vehicles, servicing costs between diesel and petrol are about the same, as long as you don’t double the frequency of the recommended service interval.
Some people have gone to the trouble of servicing their diesel vehicle every 5 minutes but have been unlucky and had injector problems or valve clearance problems or something similar. They’ll tell you that modern diesels have excessive maintenance costs. However some people with petrol vehicles get unlucky too. I think, on average, maintenance will be less on diesel. They are built tougher and generally last longer (see Longevity and Reliability section below).
I’ve heard people say “get a drop of diesel on you or your clothes and that’s it. You’ll stink for the rest of your life.” I’ve never had this problem. It’s true that diesel is less volatile than petrol and so will hang around longer. But I think you’re clutching at straws if you’re using this as an argument against diesel. It’s so insignificant. I have never noticed that I smell like diesel. There’s millions of people using diesel without complaining. Use other factors to determine your preference.
Because of diesel’s universal use in industry, mining, agriculture and power generation, it is found everywhere. Wherever there are people, there’s diesel. For example, some remote communities stock diesel only. Homesteads stock diesel only. Mine sites stock diesel only. Any remote area that needs power will probably get power from diesel gensets, so diesel will be found there. If you’re going bush then diesel is a better option in terms of availability. Having said that, most communities will have petrol so the difference isn’t huge.
There are more cars available with petrol engines than diesel engines. The car you want may not come in diesel. The diesel variant may be a longer wait. So in terms of shopping convenience and available options, petrol is better. In terms of second hand availability, because initially most cars are petrol, the second hand market for petrol vehicles is huge. You’re more likely to find a second hand vehicle to your liking with a petrol engine simply through probability in numbers.
Diesel is more efficient than petrol. This is because of a diesel motor’s higher compression. The laws of thermodynamics dictate that higher efficiency can be obtained through higher compression. So fundamentally, independent of design specifics and vehicle specifics, diesel engines will be more efficient. There is no argument. It’s physics.
Another reason that diesel is more efficient than petrol is that diesel does not need to be throttled the same way that petrol does. In a petrol engine, at anything less than 100% throttle, the engine needs to work to suck air in through the throttle body. The restriction means the engine works harder on the intake stroke and thus looses efficiency. In a diesel engine there is no air throttling.
Another reason why diesel is more efficient than petrol is that diesel can run extremely lean. Petrol can’t run too lean because lean mixtures cause engine knock / pinging and extreme temperatures. Lean mixtures yield more complete combustion. More air also means there’s a greater amount of gas to absorb the energy of combustion. More energy goes into increasing pressure and less energy goes into increasing temperature. This also improves efficiency.
At high loads the lean advantage of diesel gets even bigger. Engine knock / pinging becomes an increasing problem as load increases in a petrol engine. So at high loads petrol engines need to run even richer than they normally do. A diesel engine on the other hand can always run very lean no matter the load. At high loads the mixture is maintained for peak efficiency. There is no compromise made to control pinging like in petrol engines.
Another reason diesels are more efficient is that at low rpm they deliver more power and torque. This means, for a given power requirement, a diesel engine will be revving lowing than a petrol. Lower rpm yields less friction losses.
People may flag examples where they believe a petrol engine was more efficient than a diesel engine on a particular trip, or that the difference was not enough to warrant diesel. There are variables that haven’t been accounted for when a claim like this is made. For example – loading, driving style, tyres, etc. There would be reasons other than engine efficiency to explain it.
When comparing fuel efficiency you should look at figures published by manufacturers. Although this may not represent how much fuel you will actually use, it provides the best information for comparing because the testing is based on a strict standard. Variables have been controlled and accounted for. Don’t go by some guy’s story about his petrol using less fuel then his mate’s diesel. If the manufacturer quoted figures indicate the petrol uses 50% more fuel than the diesel then you can expect that difference in real life. Your driving style, vehicle loading, combination of city and highway, terrain traveled, vehicle modifications etc will yield a fuel consumption that is not the same as that provided by the manufacturer but the difference if you changed engine types would still be consistent with the figures provided by the manufacturer.
Lets say you don’t care much about the improved fuel efficiency of diesel. Maybe the lower purchase price of petrol is more important. There’s still an advantage to having a more efficient diesel: improved range between fills. For a given sized fuel tank, a diesel engine will go further than a petrol engine. This becomes more important for remote area travel. With a petrol engine you may not be able to carry enough petrol to achieve your required range. With petrol you will also rely more on remote area service stations where the fuel can be very expensive. You will also spend more time stopped at service stations and so your transit time will be longer.
Longevity and Reliability
One of the reasons for a diesel engine’s universal appeal to business is its longevity and reliability. Diesel engines last longer and are less prone to break down, particularly when short trips are avoided. There are several reasons for this. One is that diesel engines usually have lower specific power than petrol engines. They produce less power for a given engine size. This means the engine is under less stress. Diesel engines are built stronger to withstand the higher compression, so for a similarly sized engine they are stronger and produce less power, giving a doubly good improvement to longevity. Diesel fuel is self lubricating. This means the upper parts of the cylinder are lubricated by diesel, particularly on cold starts when engine oil hasn’t had time to reach the top of the engine and the fuel mixture is rich, which can wash some oil away. This small improvement in lubrication accumulates over time to help yield a longer living engine.
Diesel burns cooler than petrol which means diesel engines run cooler. This yields less heat load and less fatigue and wear due to heat.
Diesel engines usually have more torque and power at low rpm. This means, for a given power requirement, they can rev lower than a petrol engine. The lower rpm of diesel yields less wear and longer life.
In terms of reliability, one thing clearly gives diesel the upper hand – the lack of an ignition system. An ignition system does not exist in a diesel engine, completely eliminating that failure mode. Generating and propagating high voltages in small spaces is inherently unreliable. The system is sensitive to the effects of dirt, moisture, gap clearances, wear and insulation degradation. There is no way around it. High voltages are difficult to control with the precision that a petrol engine demands. Many roadside breakdowns in petrol engines are related to the ignition system. Resistance to water favours diesel for the same reason – the lack of an ignition system means splashes of water or extreme humidity are less likely to stop the engine when compared to petrol.
Some people believe diesels are too expensive / unreliable due to modern high pressure injectors. Eventually the injectors fail and they are expensive to replace. Yes some people get unlucky with injectors, but generally injectors are reliable and, over the life of a vehicle, will cost less than the extra fuel consumed running a petrol engine. Additionally, as common rail technology matures, the injectors are becoming more reliable and cheaper to replace. Soon the injector issues of first generation common rail technology will be forgotten and this argument against diesels will no longer exist.
I’ll share my personal experience with injectors as an example. My injectors needed replacing at around 190,000km and they cost $2,500 to replace including parts and labour (I would have done it myself and saved a thousand bucks if I wasn’t travelling at the time). Using ADR fuel economy figures (8.3L/100km for diesel hilux vs 13.1L/100km for petrol hilux) I can calculate roughly how much extra fuel the petrol would have cost me over 190,000km. The difference is 4.8L/100km so the extra fuel is 190,000 / 100 * 4.8 = 9,120 liters. At roughly $1.50 per litre, that equates to $13,680. Deduct the cost of the injectors and my diesel has saved me $11,180 over 190,000km.
The above calculations do not consider my reduced reliance on remote area service stations. The long range provided by diesel means I can often wait to major service centers to refuel rather than relying on expensive fuel in remote areas. So the money savings are even more than calculated.
The above saving discounts any difference in engine longevity. Hopefully the diesel engine will last 500,000km or more. Sometimes petrol engines need a complete rebuild at around 200,000km. That’s roughly the same mileage my injectors lasted. So in a petrol engine you may be facing the cost of an engine rebuild instead of the cost of new injectors. Or if a petrol engine would last 300,000km then at the time a diesel’s injectors are replaced you could say you’re up for roughly 2/3 the price of a complete engine rebuild if it were petrol.
Power and Performance
Petrol engines generally have higher specific power than diesel engines and so, for similarly sized petrol and diesel engines, the petrol will usually have significantly more peak power. They are also cheaper to make, so that the same model vehicle will likely have a bigger capacity petrol engine compared to the diesel variant in order to keep the price differential within certain limits. So, if you want maximum performance, a petrol engine is better.
The problem with petrol engines in terms of its fun factor is that they are quite dull at low rpm. You need to rev them up to get the power. Diesels on the other hand generally have more low end torque and power. This means a diesel engine can feel more fun than a petrol engine even though the diesel’s peak power might be lower. For example you can give the throttle in a diesel engine a bit of a squirt at low rpm in second gear around a corner and it will feel fun. The engine feels strong and responsive at low rpm.
Diesel’s greater low end power also makes them a superior tow vehicle. You get more power down low so you can effortlessly tow without high rpm. There will be less gear changes and speed will be maintained better up hills.
A diesel is superior in most offroad applications. The greater low rpm power and torque requires fewer gear changes and less rpm to conquer obstacles. For very steep, uneven and rocky terrain a diesel engine allows you to go slower and with less speed variability. You can chug away at very low rpm with practically no changes to throttle regardless of whether climbing a rock or dropping down a large step. It’s easy to power through deep sand without needing to shift gears. Diesels are also at less risk of problems during water crossings. The high voltage electrical system is easily disrupted by water.
The higher peak power of petrol engines can be an advantage in some offroad applications. For example hooning around dune systems or high speed attempts at crossing muddy bogs.
Because of the long life of diesel engines, they generally hold their value better when compared to petrol engines. So although the initial price is more for a diesel, the gap is closed when it comes to selling and you take into account the net capital outlay. I think petrol may still have the upper hand, but not by much.
In Australia in big cities, petrol is usually cheaper than diesel. Diesel is a few % more expensive, possibly wiping out any potential fuel savings afforded by greater efficiency, or does it? As an example, the ADR efficiency for a 2015 4WD petrol Hilux is 13.1L/100km compared to 8.3L/100km for the diesel. The petrol uses about 58% more fuel. That is a big difference. Even with diesel costs being slightly higher, the diesel is still significantly cheaper to run. The ADR figures may not indicate what you will actually use, but because the test is under controlled conditions the comparison between figures is fairly reliable.
In remote areas the gap closes and diesel is about the same price as petrol and sometimes cheaper. I guess it’s because of the greater supply of diesel to remote areas. So if you go bush often diesel may be simultaneously cheaper and more available.
In the military, diesel engines are preferred for armoured fighting vehicles. One of the reasons for this is the reduced flammability and enhanced safety of diesel fuel. Under anything less than ideal conditions, diesel pretty much wont burn. Chuck a match in a pool of diesel and nothing will happen. It may burn slowly if you apply very high heat, for example from a blow torch, but will most likely put itself out once the heat is removed. Diesel only becomes explosive when atomized or vaporized at high pressure and temperature. Petrol on the other hand will always burn quickly and violently. So if you’re carrying lots of fuel for long bush missions through the outback, then diesel is a safer option. Especially when mucking around with jerry cans and refueling under less than ideal conditions on the side of the road. A bit of static or electrical spark will have you quickly incinerated with petrol.
Another reason that diesel is safer is that, because it burns cooler, the exhaust system is much cooler. This reduces the chance of fires caused by exhaust contact with flammable material (for example dry grass).
Like everything in life, there is no perfect answer to petrol vs diesel. There is always compromise. If it was perfectly clean cut then the less favourable alternative wouldn’t exist. If you’re doing high kilometers or you’re going bush a lot, then diesel might be a better option. Otherwise the lower purchase price and better performance of a petrol may be of an advantage. If you’re towing something then diesel may be better, as it should last longer under constant heavy load and the fuel saving will be more pronounced. However in any application either option will provide an adequate solution.
If you want to commute to work and go to the shops, then diesel is definitely not the best solution. The high purchase price and extra wear on the engine and emission systems due to short trips will blow away the meager fuel savings. It’s the wrong tool for the job. A better tool would be a pushbike or scooter, or at most a small petrol powered car. If you want to haul you and your gear hundreds of km then diesel is a good solution.
I started on petrol as a city boy going for occasional camping and fishing missions. For touring around Australia I went with diesel and so far I think it’s been a better choice. Fuel costs are easily the single biggest cost when pulling huge kilometres on a tour around Australia. The fuel savings are great, and so far I’ve had exactly zero problems with the engine, other than the injectors at 190,000km.