Are diesel engines more efficient than gas engines?

Oil

The main criteria of a car’s eco-friendliness is generally seen as the energy it uses in its operation. Cars have primarily run on oil-based fuel in the form of petrol or diesel for over a century. But we are now at the point where demand for oil is rising so much, and reserves are declining, that oil is actually running out. Oil prices have always been volatile and they react to speculation about events ranging from terrorism to hurricanes, but rising demand and diminishing reserves mean that overall, oil is just going to keep on getting more expensive. However despite this, most cars today still rely on oil as their primary source of energy; and oil is obviously a key contributor to carbon emissions, and climate change.

Petrol

Petrol has been the most popular fuel for cars in the UK for years; petrol engines are generally quiet and smooth, they are responsive and their performance is good. Petrol is currently slightly cheaper than diesel. Petrol engines emit around 10% more carbon dioxide (CO2) than diesel. However petrol cars pump out less toxic emissions than diesel. Unfortunately at the moment there is no single source of fuel which can compare with petroleum in terms of its instant bulk availability, energy density and (relative) cheapness.

Diesel

Diesel engines are more economical than petrol engines, therefore they emit less CO2. New ‘common rail’ diesels are approximately 10% more efficient than older diesels, and direct-injection diesel engines give the best fuel economy, diesels emit more particulates than petrol – but diesel engines with a particulate trap help prevent emissions of sooty particulates – ie. the clouds of smoke that you’ll experience if you follow old buses through towns. So diesel engines will generally provide you with more miles per gallon than their equivalent petrol models – just look at the differences between similar vehicles in our Green Car Guide. Diesel is currently more expensive to buy than petrol, and the forecasts are that diesel prices will continue to rise more steeply than petrol in the near future.

Diesel engines have always been seen as slow and noisy, however technology has seen some remarkable advances in recent years; for instance Honda has developed their own diesel engine that is designed to be quiet, refined, clean and with instant response – fighting against all the old stereotypes.

LPG

Over recent years, LPG (liquefied petroleum gas) has been a viable fuel option in the UK. LPG produces fewer emissions than petrol and diesel but fuel consumption is worse. It’s been possible to convert many existing cars to run on LPG by after-market conversions, and some manufacturers such as Vauxhall have had new cars in their range that are dual-fuel, which are designed to run primarily on LPG with petrol back-up. There is a reasonable network of filling stations.

LPG, and natural gas in heavier vehicles, has been an attractive proposition in the past primarily due to its cheaper cost, as it has enjoyed less fuel duty. However there is no guarantee that the Chancellor will maintain this in the future, and although there are some emissions improvements over petrol, LPG is still derived from a fossil fuel and therefore still releases greenhouse gases into the atmosphere.

CNG

Some vehicles, usually heavier vans or trucks that normally run on diesel, but also cars such as the Volvo (V70 Bi-Fuel), can run on CNG (Compressed Natural Gas), which again results in lower CO2 emissions than standard petrol cars, but the fuel is not as efficient as diesel. Finding CNG for refuelling can be a challenge.

Petrol-Electric Hybrids

Petrol-electric hybrid vehicles run on a combination of a conventional petrol engine and an electric motor powered by an energy storage device such as a battery pack. In simple terms they work on the principle that an electric motor provides the power at low speeds such as in urban driving, and they switch to petrol for driving at higher speeds. The batteries are recharged while driving and hybrids use regenerative braking, which means that energy is put back into the battery when braking, which improves energy efficiency.

Hybrid technologies improve fuel efficiency and therefore provide considerable fuel savings compared with a normal petrol vehicle – as well as carbon emissions savings. While models might cost more than conventional cars, running costs can be two-thirds that of equivalent petrol-fuelled vehicles.

Because of their lower CO2 emissions, hybrids also benefit from reduced vehicle excise duty and are treated favourably in Budgets. In addition they are exempted from the London Congestion Charge.

However at the moment there are a limited number of hybrid vehicle choices; there are currently just four hybrids available in the UK; the Toyota Prius, Honda Civic hybrid, Lexus RX400h and Lexus GS450h. As they are still a relatively new technology, there aren’t many available second-hand and so they are quite expensive.

Toyota’s first Prius (launched in Japan in 1997) didn’t sell in great numbers, however a new model has been introduced and this is now proving more successful. Although it looks like a normal car, it is designed around energy efficiency, and has many clever technological features that assist fuel consumption, including air conditioning and brakes powered by electricity rather than by sapping energy from the petrol engine. Lexus, part of Toyota, has introduced a hybrid version of the RX300, known as the RX400h. This is an SUV and because of its size, it still only returns around 35mpg compared to the Prius’s 65.7mpg.

Small diesel generators sometimes have a bad image. But in my opinion that is just a prejudice. It is a well known fact that diesel engines are the most preferred engines in industrial settings. And that has been the case for a great number of years. There is very much experience with these engines. So you will understand that very important issues like cost, reliability, and efficiency have been thoroughly evaluated. Therefore it will be no surprise to you that diesel scores good at these issues. Diesel is more reliable, more efficient, has a longer life cycle and require less maintenance than engines that run on other fuels. And this is not just the slogan of a commercial, but there are solid technical reasons to back this up.

An important technical issue of internal combustion engines is that a large part of the potential energy from the fuel is converted to heat in stead of useful power. And on top of that some of that power has to be used for cooling the engine to prevent overheating. The temperature of a running diesel engine is much lower than that of any other internal combustion engines, so a greater part of the energy from the fuel is converted into useful power and less of this power has to be used for cooling.

But that is only part of the reason of the efficiency of diesel. Another reason why these engines are more efficient is because the compression ratio. The compression ratio of a diesel engine is 14:1 up to 25:1 and the compression ratio of a gasoline engine is between 8:1 an 12:1. And the higher the compression ratio, the more efficient an engine will be.

Because of the efficiency the cost of using a diesel engine will be less than that of a gas engine. To be more specific: the fuel cost of a diesel generator per kilowatt is 30% to 50% lower compared to a gas generator.

And of course engines require maintenance. And that is yet another reason for the popularity of diesel engines: they require less maintenance and also last longer than gas engines. Partly this due to the lower temperature of the diesel engine we discussed before. But another reason is for the reliability and the lower maintenance cost is the fact that there are no spark plugs or spark wires in a diesel engine. So these parts cannot cause problems will not have to be replaced regularly.

The last prejudice I want to talk about is the fact that many people think that diesel engines are very noisy and smelly machines. And indeed that was true – in the past. But not any longer, because the past decades diesel technology has made great progress. Diesel engines run quieter and less noisy then ever. Better insulation material have been developed (and used). And also the use of mufflers has contributed to the low noise diesel engine.

Considering a turbocharged vehicle? Big kahuna intakes or parallel twin turbo? That’s not important for now if you have a shoestring budget. The first thing you should consider is how fuel efficient is it? Then whether gas or diesel. Read on!

Turbocharging is the method with internal combustion engines to use a vehicle’s exhaust gases to cram force-induced compressed air into the engine. This concept follows the idea that a normal internal combustion engine’s ability to use air to power a combustion cycle is limited to the negative pressure its pistons creates to draw air in from the outside unaided.

However, with the turbocharger, more air mass is forced into the engine, thereby making the engine burn much more efficiently, and hence performs much better.

Comparatively, turbochargers are used more often in diesel engines due to the following characteristics:

-Normal diesel engines are designed to be over-built, helping it withstand higher pressures and heat from the start.

-Diesel engine power outputs are less compared to gasoline engines which require less pressure from turbochargers to produce more power.

-Diesel engines are not limited by the octane ratings of gasoline as diesel engine are designed to pump fuel into its combustion chamber only at the moment of ignition.

-Are proven to be fuel-efficient than diesel engines with no turbocharger. In diesel engines, more air compressed into the engine results in greater fuel efficiency.

Practically, there is little downside to owning a turbodiesel vehicle.

Turbocharged gasoline engines, though, may perform better than turbodiesels but require more consideration for balance and maintenance.

On the other hand, installing turbochargers in gasoline engines have the following caveats:

-Gasoline engines are lighter than diesel engines. Forcing air into engines has the added effect of “overclocking” the engine, increasing heat and pressure above normal specifications.

-Adding a turbocharger to a car is more expensive than buying a car with a stock turbocharger as upgrades to other parts of the car are needed to bear the added heat, pressure, torque and power.

-Gas turbochargers have to take into account the octane ratings and autoignition temperatures of its fuels.

-Gasoline engines spray gas and air into the combustion chamber at the same time it is being compressed. The act of compressing the air with already compressed air from the turbocharger may detonate a low octane gas prematurely.

-Maintenance is high, which limits gas turbochargers to high performance vehicles.

So how can I turbocharge my vehicle more fuel efficient without spending so much more?

Buy a turbodiesel. Hands down turbocharged diesels have the advantage by the price of diesel nowadays, the very design of diesel engines, and the way it saves diesel the more air you pump into the engine! Turbodiesel vehicles have a good resale value so don’t expect rock bottom prices, but expect years of good use from even an old turbodiesel. Best value for money!

Buy gasoline vehicles with turbo already installed. Face it. Unless you’re a car enthusiast, installing a turbocharger in a car will cost you too much!

Buy smaller gasoline vehicles with turbo installed. There are smaller vehicles in the market that sell units with turbochargers to maximize the small size of the engine. Look up units in the Volvo and Saab lines.

Consider alternative fuels. You can use alternate fuels such as a hydrogen-on-demand (HOD) system to supplement a smaller turbocharger and engine tandem. This system creates hydrogen (H2) and oxygen (O2) gas on demand from a water medium to mix with the air and fuel in the combustion chamber for even better performance.

What does a HOD system do?

Hydrogen when compressed and introduced to an internal combustion engine:

-Releases more energy than ethanol-based mixes, maintaining the overall power of the car

-Burns the fuel more completely which gives cleaner emissions

-Raises the octane level of the fuel, preventing knocking, making the engine quieter

-Lowers overall engine temperature slightly

-Requires the engine to use less gas per cycle

Is it safe?

HOD systems generate hydrogen only on demand and are therefore safe. When no electricity is routed through the HOD system, no reaction is generated. No hydrogen is in storage in this system but is kept in medium, which is basically water.

Cost?

Parts are sourced from everyday parts found in hardware shops and basic electronics stores. Total cost for a basic unit can range between $70 to over $200 USD.

What’s offered by most HOD enterprises are plans for do-it-yourself systems, with offers to build the system for the consumer if they so wish. This way the customer can opt to build it himself or to buy a unit once the concept is understood after obtaining the plans to prevent fraud.

Better Fuel Economy: Diesel engines use compression ignition instead of spark plugs to start the combustion process. Using compression as the ignition source, combined with the higher energy content of diesel fuel, provides a much more efficient engine which requires less fuel to operate than a gasoline engine. A diesel engine is also able to operate at peak torque at lower RPM, like you would typically see at cruising speeds, further boosting fuel economy.

More Power: A turbodiesel engine can produce much more peak torque than a gasoline engine of equal size, and maintain peak torque over a broader RPM spectrum.This provides for much more power throughout the operating range, less downshifts for improved fuel economy, and increased payload ratings.

Higher Towing Capability: Due to the stout structure and additional torque of a diesel engine, payloads on diesel powered trucks are much higher than those on similarly sized gasoline V8 powered trucks. If you have something large to tow, a diesel will make the task much easier and provide the best fuel economy possible while loaded.

Aftermarket Support: Diesel powered trucks are receiving much more attention from the aftermarket than gasoline powered trucks due partly to the fact that diesel engines are easy to tune for more performance. Where a few simple bolt on parts may increase your gas powered truck’s performance by 50-75 horsepower, the same parts could take your diesel to extreme performance levels. How does an additional 200-350 horsepower sound? (Based on performance testing of a tuner, cold air intake, and exhaust system; the three most common and readily available engine modifications.)

Driving Experience: If your any sort of automotive enthusiast, or just like a truck that can run through the gears fast, diesel is the way to go. Just remember to hang on once the turbocharger spools, because you will be in for one wild ride. Diesel engines produce gobs of torque at low RPM, which pushes you back in your seat as the truck takes off when you start getting into the throttle.

Engine Longevity: Diesel engines operate at lower RPM and have much stronger engine parts than comparable gasoline engines. This translates into increased engine life, with diesel engines generally lasting up to 2x longer than gasoline engines before needing an overhaul or major repairs.

Cleaner Emissions: New technology has cleaned up the diesel combustion process, reducing the amount of harmful greenhouse emissions and virtually eliminating diesel soot from the exhaust.