The Bigger Bang: Diesel or Petrol? --Research
Users of Motorsport Forums! This is the first time I have used a forum of any sort, so please pardon me if anything about this post seems peculiar. I have been involved in a research project at school; my topic has been the use of diesel engines in motorsport. I am now looking for feedback on an article that I have made briefly summarizing my findings. Any and all comments are helpful and appreciated.
Ross Carlson
The Bigger Bang: Diesel or Petrol?
For years, the automotive world has been centralized around increasing performance, and increasing efficiency. Recently, automotive manufacturers have been using new technologies to aid in these fields. It’s fair to say that nearly everyone knows the impeccable fuel mileage that the Toyota Prius is capable of attaining, and in Formula One, the amount of power produced per liter is the highest it’s ever been, despite the many regulations imposed by the Federation Internationale de l’Automobile. However, this industry has also been constructed upon the foundation of one particular fuel: petrol. For many years, automotive manufacturers have been producing diesel powered cars, often these cars were considered a inferior to petrol powered cars; they were heavy, slow, loud, and smelly. Luckily, technology has reduced these rather annoying properties to a quite tolerable level. Today about a third of all cars in Europe are diesel powered, and for a good reason. The Volkswagen Polo, sold in Europe, is capable of 99 miles to the gallon, a stunning feat; the most fuel efficient petrol powered car, the Kia Rio, is only capable of reaching 88 miles per gallon. So with this profound difference in the regular automotive sector, what new occurrences have come about in the performance sector?
To answer the previous question simply, diesels haven’t been especially revolutionary in motorsport. In recent years the Audi R18 has provided an interesting prospective at Le Mans, placing very high in the ranks, and even winning some races. Earlier in 2009 Volkswagen’s Touareg won the DAKAR series in Argentina.
What it comes down to is three things, more energetic fuel (high specific energy), more favorable power and torque bands, and safety.
The safety of each fuel can be determined based on the flashpoints. Diesel fuel does not burn unless it is very hot. In the unfortunate event of a crash, one of the main concerns is that the driver and the cockpit remain free of fuel to prevent burns and other heat related injuries. Drivers already use fire retardant suits, and sensors will shut off valves to the well protected fuel tanks to prevent any chance of fire reaching the driver, however, in some cases fuel, in the engine compartment of the car, is still left over and can ignite outside of the engine. Firewalls prevent the fire from reaching the cabin, where the driver sits, but still poses a hazard. If diesel fuel were to be used, spilled fuel, and fuel left in the engine compartment would be less of an issue; the high flashpoint will not permit the diesel fuel to ignite in open air, increasing the driver’s safety by magnitudes. Diesel fuel is also less volatile, meaning it does not emit as many fumes as petrol. Fumes and vapors are much more likely to cause explosions than liquid fuel. You may have noticed that at gas stations, there is not only a fuel nozzle, but a vent pipe as well. In petrol cars, any shaking or increased temperature will likely evaporate some of the petrol and create pressure on the seal. This vent removes the fumes created by the shaking, and moves them into the large tanks beneath the station. Most diesel nozzles do not have this vent, this is because few diesel vapors are present in the tank - most is ambient air. When the tank is filled up, the air is expelled from the tank, into the surrounding area. The thicker composition of diesel fuel is less volatile and produces fewer fumes, so the risk of any fire and explosions is greatly reduced.
Looking further into diesel fuels’ high specific energy, figures for both diesel fuel and petrol can be compared. Gasoline, petrol, has a lower specific energy around 44.4 MJ/kg, this fuel uses smaller hydrocarbon chains, containing mostly paraffins, the chemicals present are C8H18, C4H10, C5H12O, and C9H12. These have only single bonds between the carbon atoms, and it is these bonds that supply the chemical potential energy. The more bonds broken during combustion, the more energetic the reaction. In diesel fuel, the specific energy is much higher, around 45.4 MJ/kg because the chains of hydrocarbons C12H23, along with some C10H20 and C15H28 are longer, and contain more covalent bonds between carbon atoms to break. These chemicals are high density and can be found in kerosene (C12H23), ether (C10H20), and castor oil (C15H28).
Power is force in relation to velocity, or work done divided by time, and is represented by and individual point on the torque band. It is found by multiplying torque by the rotational velocity at that point and a constant. However, the highest instance of torque is not necessarily the highest point of power. For example, if a simple lawnmower engine provides the most torque, around 30 ft lbs, at 2000rpm, and provides half as much torque, 15ft lbs, at 6000rpm, the latter instance provides more power in the system. 30*2000 < 15*6000, worked out, 60,000 < 90,000. What this does mean though, is that in any gear ratio, the engine will accelerate the car fastest, at the highest point of torque, but only for that gear. If the ratio is taken down, trading rotational speed for torque, the higher power will accelerate the car faster than the previous situation, even though the torque is less. To be clear, here is a comparison of a torque band, in yellow, from a VW 1.9L Turbo Diesel Engine (top) and a torque band, in blue, from an extremely similar VW 2.0L Turbo Diesel Engine (bottom).
[img]file:///C:/Users/Leslie/AppData/Local/Temp/msohtmlclip1/01/clip_image002.jpg[/img] [img]file:///C:/Users/Leslie/AppData/Local/Temp/msohtmlclip1/01/clip_image004.jpg[/img]
In both torque bands, it is seen that the max torque occurs around 1900 rpm, but the max power occurs at around 3000rpm, in blue on the 1.9L and in yellow on the 2.0L. The turbochargers used on each changes how much torque each engine produce. The turbocharger on the 1.9L engine applies more pressure on the piston during the intake stroke pushing more air into the cylinder, so more fuel can be injected, and therefore more power and torque can be produced.
The most favorable power curves have a very high mean power over a large range of rpm. Petrol engines tend to have a power curve that peaks at the max, but has a mean power that is very low. Diesel engines generally have a very level power curve through almost 50% of the curve, from 2200 rpm, to 3200 rpm out of the full range from 1000 to 3200 rpm. [img]file:///C:/Users/Leslie/AppData/Local/Temp/msohtmlclip1/01/clip_image006.jpg[/img][img]file:///C:/Users/Leslie/AppData/Local/Temp/msohtmlclip1/01/clip_image008.jpg[/img]The 2.0L VW petrol engine has a less favorable power curve as it peaks at around 5000 rpm. The equivalent usable curve is from 4200 rpm, to 5700 rpm from the entire curve from 2400 rpm to 6100 rpm, so the equivalent band only uses 40% of the total band. It is obvious that the area under the curve for the diesel this specific range is larger than the area under the curve for the petrol engine. However, the petrol engine has a major advantage over the diesel engine. Because the actual combustion of diesel fuel is less powerful than gasoline, the engine itself is not as powerful, and this shows in the magnitude of the bands. The peak power of the diesel is 63 kilowatts which is about 85 horsepower. The peak power of the petrol engine is 106 horsepower, about 79 kilowatts. The petrol engine is capable of achieving 20% more power than the diesel engine; an outstanding result.
Diesel Engines tend to have more favorable curves, but have a much smaller magnitude of power. Technically, the engine produces a higher mean value of power throughout the range of rpm, but in full stature, the peak power of diesel engines is much less than the peak power of petrol powered engines. Higher mean power is a very good attribute when constantly accelerating and decelerating on a very curvy rally circuit, but is useless when the goal is top speed on long straights.
There is also some error within the graphs, and what trends occur between manufacturers. These differentiations are caused by the different styles of other systems among engines. In this case, the data can be considered reasonable, but unreliable since the diesel charts being examined are industrial engines, not intended for performance use.
At the present time, petrol engines are more suitable for motorsport, however because the diesel engine is superior in principle, research and development should be conducted in order to advance its effectiveness. Informing amateur motorists of diesel fuel capabilities will lead to more efficient vehicles on the road in general, and will encourage the research and production of diesel engines for performance use. The science behind engine mechanics has been studied since the internal combustion engine was created, in all, it is very extensive, and elaborate, still today research is being conducted to determine what new capabilities the internal combustion will have, from high performance exotic fuels, to safety in the inevitable.
Images from:
Volkswagen. Marshall's Industrial Products. Rep. 2011. PDF. Specifications Marshall's Industrial - Volkswagen powersystems and components distributor for the UK. IMPCO Gas products Distributor
