Petrol or Diesel? It should be a simple answer to a simple question. Unfortunately there is no one place for the answer. Here are the facts I can find.
Ethanol is a commonly used renewable alternative to petrol. Diesel has a higher boiling point than petrol and petrol has a higher boiling point than ethanol. Ethanol is currently not a safe choice in extremely hot areas because of vapour formation. Ethanol works at a much lower temperature than diesel and is a good choice for extremely cold areas. For everywhere in between the two extremes, you can safely use diesel and petrol plus the renewable alternatives of ethanol and vegetable oils.
Small vehicles have a huge choice of engines. Very large vehicles often have only diesel engines. This page compares fuels for vehicles where there is a choice of equivalent engines.
The only current alternative for very large vehicles is to replace part or all of the diesel with vegetable oil. Rudolph Diesel invented the diesel engine and the fuel he used when he first publicly demonstrated the diesel engine was peanut oil. The only current problem restricting the use of vegetable oil in diesel engines is the formation of water and sticky wax in cold weather.
In Australia about 10% of our population would not be able to use vegetable oil in winter without heavy processing of the oil. North American vegetable oil usage would be minimal in Canada and perhaps half of USA. Companies are working on both better refining of vegetable oils and fuel warming systems for cold areas.
The media like to tell you just one side of a story because it cuts down on research costs, makes articles smaller, and leaves more room for advertising space they can sell to people who are pushing a product as
green. The media make a profit from telling you the side of the story favourable to their biggest advertisers. The media tell you about carbon dioxide causing greenhouse warming and diesel cars use less fuel than petrol cars. They do not tell you that carbon dioxide is only one of several human made greenhouse gasses, that there is ten times more carbon dioxide added to the atmosphere by tree clearing than from cars, or that diesel produces more carbon dioxide than petrol.
Diesel is the same worldwide. Petrol varies in carbon density. Diesel produces 10% more carbon than petrol based on the worst of the common forms of petrol. All the carbon from diesel is unnatural and increases the greenhouse effect. Many countries add some ethanol to their petrol and the carbon in ethanol is recycled carbon from plants. Some cars in Brazil use 100% ethanol, a cleaner approach than any other choice.
If we compare cars based on pure petroleum based petrol, diesel beats petrol by a tiny amount and hybrid petrol cars beat diesel. As a comparison in Australia, Ford advertises one of their diesel cars as using the smallest volume of fuel per distance travelled but a Toyota hybrid car produces less carbon per distance travelled because petrol contains less carbon.
There are currently no hybrid trucks in large scale use. There are several types in testing phases. Diesel beats petrol by a small margin for large vehicles and petrol hybrids beat diesel for small to medium vehicles.
The most common ethanol-petrol blend is 10% ethanol. Brazil has 100% ethanol fuel on sale and Australia is about to start selling 100% ethanol fuel. Ethanol has a lower carbon level than petrol and you burn a little more fuel when burning ethanol blends. The 10% ethanol blend has 10% less polluting carbon but you burn more and the result is you produce 7% less carbon pollution. Combine that 7% with the already 10% less carbon from petrol, compared to diesel, and you get 17% less non renewable carbon in the ethanol-petrol blend compared to diesel. Based on current car technology, petrol cars using 10% ethanol produce less non renewable carbon pollution than diesel. This is true across the range for all vehicle sizes where there is a petrol equivalent.
Petrol-ethanol blends beat diesel for cars and small trucks.
Diesel produces 24 times more nitrous oxide than petrol according to several tests with current vehicles. Cities with air retention problems need to limit diesel use. Los Angeles and Sydney are two classic examples of cities in basins that trap pollution for days at a time. Nitrous oxide pollution is a killer in the heat of summer and in damp foggy winters. There is no way to cure the diesel nitrous oxide production problem because diesel engines work at a fixed high compression. Diesels are a bad choice for some cities.
Particulate matter is the grubby poisonous carbon smoke from diesel. Diesels are many years behind petrol engines in this area. This is another reason some cities want to minimise diesel usage.
There is no carbon smoke from modern cars when maintained as new. The cars will likely end up the same as trucks when they reach the second hand market and the new owners are hit by the extra high maintenance costs.
Diesel engines are heavier than petrol engines for the same power output. The weight difference is not a problem in large trucks, ships, and trains. The weight difference is significant in cars and is another reason why diesel cars do not easily beat petrol engine cars when you compare two cars with the same carrying capacity.
Ask a family loading up their car for a trip to the beach. The impressive low diesel fuel consumption from the advertising is based on an empty car on a flat road. You put the same heavy diesel on a steep hill and have to press the gas pedal down hard to push the car up the hill. After you add people and luggage, the trip becomes painfully slow.
Petrol engines have a wide power range. Diesel engines have a narrow power range. You have to use extra gears to keep the diesel in the power band. If your petrol engine car has a six speed gearbox, you need a 7 or 8 speed gearbox for an equivalent diesel. The extra gears add more weight to the vehicle. Again it does not matter in a truck weighing close to 50 tons loaded. Where it does make a significant difference is in a small family car. The last time I checked the specifications for two otherwise identical cars, the extra weigh of the diesel was equivalent to a small adult or several large items of luggage. This is weight you drag around all the time and it increases your fuel consumption.
This comparison uses litres and Australian dollars. The following prices are common on the most expensive days of the week. Both the standard unleaded and the ethanol blend are more than $0.10 cheaper if you buy during off peak periods. Diesel and the premium octanes do not vary across the day or week.
10% ethanol blend petrol: $1.27
Standard petrol is unleaded and referred to as ULP: $1.31
Premium 95 octane: $1.48
Premium 98 octane: $1.52
All the diesel cars cost more than their petrol equivalents. There are many published comparisons showing close to identical cars with the diesel cars costing thousands of dollars more. The compared cars are often European and the Europeans use an expensive premium petrol. Despite the huge cost overhead of the premium petrol, the European diesel cars cost more to own than the European petrol equivalents. If you buy a petrol car, you can save enough money to buy green electricity for your house and save more carbon pollution than your car would ever save.
If you ignore the European trash cars with their inappropriate engines and buy something designed for Australian conditions, you will be able to use the ethanol blend and save many more dollars heaped on top of the reduced carbon pollution from the ethanol. All the extra money saved from not buying a diesel engine and not buying a car requiring a premium fuel is enough to fit your house with a solar power system.
You can see where this is headed. Buy a car designed for your local conditions, not some snobby little brand designed for a cold little town in Europe or an inappropriate iron hulk from America. In the Australian case, we commonly drive long distances than you would in Europe. A common weekend trip to visit friends or relatives is equivalent to driving from Berlin to Rome Friday afternoon then Rome to Berlin Sunday afternoon.
Friends in Europe say the drive from London to Paris is a significant distance. In Australia, there are many people driving that distance as a daily commute. For a while my daily commute was the equivalent of London to Paris then back to London then back to Paris. 15000 kilometres per year (9300 miles) is used to calculate the cost of running a car but many cars are sold after three years have anywhere from 80000 kilometres up to 250000 kilometres of use, giving real life figures of 26600 per year up to 83000 per year. 15000 kilometres is what you might record on your second car if you have two cars.
Given the use of very short distances and incredibly high petrol prices when calculating the cost of running a car in Australia, diesel cars still turn out to be more expensive. Plus the media quote maintenance costs for diesel as similar to the maintenance costs for petrol engine cars but that occurs only during the initial honeymoon period when new cars do not need significant maintenance. When you look at the total maintenance costs for your primary car, the one that does the high mileage, over several years, diesel engine cars cost twice as much as petrol engine cars.
Petrol wins on cost and the money you save is enough to convert your house to solar electricity. Petrol wins on cost even when you are stuck with a car that requires a fancy brand of premium petrol. Diesel wins on cost only when you drive a big truck and buy in bulk with a company discount.
The only cars that genuinely reduce carbon pollution are hybrid vehicles and they are currently petrol because petrol is better for the stop/start activity in a hybrid. The Toyota Prius and similar cars have a good track record in Australian cities but not in the country where they have to cruse long distances on petrol.
Some new model cars are designed to charge overnight from green electricity and last the longer commute distances common in Australia. The new designs have the potential to not use petrol or diesel during the week when you commute to work and can top up the charge at work. The only fuel use would be when you travel on holidays into the country.
Ethanol is renewable and does not contribute to carbon pollution because the carbon in ethanol is sucked out of the air by the plants making the raw ingredients for ethanol. Australia is successfully producing enough ethanol for a 10% blend. Brazil sells a 100% ethanol fuel. In Australia there is a move to process food waste into ethanol and that will let us use a 15% or 20% blend, further reducing the carbon pollution from petrol cars.
What is happening in your country? There are large parts of Africa cable of producing far more sugar than they currently produce and sugar can be converted almost direct into ethanol. When ethanol production is widespread, many of the plant sources can be converted direct to ethanol without the intermediate production of sugar, making ethanol production more efficient. Most countries have some waste vegetable matter that can be converted to ethanol.
Biodiesel is the name for vegetable oil processed for use in diesel engines. Some oils need more processing than others and all the oils need more processing for use in cold weather. One big future for biodiesel is the processing of used cooking oil. Another is the introduction of engines designed to keep the incoming biodiesel warm in winter. Biodiesel could be big in small cars where it is easy to change the engine designs quickly. Biodiesel could be big in city areas where old cooking oil is common and small diesel cars cruise the streets.
New compared to old
Some car manufacturers are adverting model X is greener because it travels further on a litre of fuel than model Y. They do not point out that the model Y is an old model with 5 gears and the model X is a new model with 6 gears. The manufacturers pretend there is a difference in the motor that saves fuel. The extra gears are tuned to fit the fuel consumption test and produce 99% of the improvement, not the engine. Do not believe a comparison unless both cars have exactly the same type of gearbox with exactly the same number of gears in the same range.
One of the problems created by ignoring the gearbox is the difference between the advertised models and the models you actually buy. The advertised models might have a 6 or 7 speed gearboxes but the bulk of the sales are the economy models with 5 or 4 speed gearbox.
One occasion when the influence of the gearbox may be different is in some types of hybrid where both the petrol engine and the electric engine drive the wheels at the same time.
Distance travelled on a tank of fuel
Some car manufacturers are adverting model X car is greener because it travels further on a tank of fuel than model Y. They do not point out that the cars have different size tanks.
Australian carbon tax
The stupid people in the current Australian federal government are introducing a new tax to take money away from the people who did not vote for them, the employed and business owners, then give the money to the voters they think they can buy, the unemployed and mums at home with children. The excuse is the new tax on carbon will do something positive for the environment. To prove the government is stupid, the government announced some of the money from their their
carbon tax will be used to subsidise the burning of non renewable resources, increasing the large scale production of carbon pollution.
Most manufacturers have little say in what they do with electricity and they will have to pay the tax. The only thing the manufacturers can do to reduce the tax is to ship their manufacturing and all those jobs to China and India where there are no limits to carbon pollution.
The people who really have a choice, the people choosing cars, appliances, lighting, insulation, and everything else for their homes, will receive massive subsidies to continue not reducing their carbon footprint. The subsidies will let people continue heating rooms they do not use and to continue every other wasteful practice. The few people who do make the right choices about energy usage will face fewer jobs and less income to pay for the switch over to more efficient practices.
If you are not yet vomiting from the bilious rubbish flooding out of parliament house, there is more bad news. The same government are going to exempt petrol from the carbon tax. The tax will not be a carbon tax, despite being called a carbon tax. The carbon tax will not be a tax on non renewable carbon or on carbon pollution. Instead it will just be an arbitrary tax on whatever the government think they can tax with only two purposes in mind, to build a massive pile of cash to buy votes, and to have a huge slush fund the politicians can use to buy whatever they want for their own pleasure and profit.
The carbon tax will not be used to subsidise the building of renewable energy supplies. There will not be a cent spent on building the conversion of food waste to fuel. Nothing for wind power. Nothing for solar power. Nothing to encourage the production or purchase of electric vehicles that could run from solar power. Nothing to reduce carbon usage. The carbon tax is a fraud.
The future of both fuels
When petrol was fist used in an engine, petrol was a natural component of petroleum but has now disappeared and has to be artificially produced by cracking oil. That cracking process wastes a lot of energy and pumps a huge amount of carbon pollution into our air.
When diesel was fist used in an engine, diesel was a natural component of petroleum but has now disappeared and has to be artificially produced by cracking oil. That cracking process wastes a lot of energy and pumps a huge amount of carbon pollution into our air.
Both fuels are doomed to rise in cost and to waste more petroleum during manufacture.
We need action now at the federal level to make cars work with higher levels of ethanol. We need action now at the federal level to make trucks work with biodiesel or blends. We need federal action now to label all cars based on their non renewable carbon emission.