Honest Auto Tech

Hello This is Bhushan. I will try to give you AutoGyan (Automotive information)/Honest Automobile Reviews.


Monday, December 24, 2018

Diesel Engine vs Petrol Engine. Why diesel is more efficient?

Hello Everyone welcome to my new blog 
While buying a new car everyone gets confused between the Diesel and Petrol motor. Will clear all your doubts regarding both. 

A brief history of Petrol Engine

The first practical petrol engine was built in 1876 in Germany by Nikolaus August Otto, although there had been earlier attempts by Étienne Lenoir, Siegfried Marcus, Julius Hock and George Brayton.
A petrol engine (known as a gasoline engine in American English) is an internal combustion engie with spark-ignition, designed to run on petrol (gasoline) and similar volatile fuels.

A brief history of Diesel Engine

Toward the end of the 19th century, the world was looking for a more efficient and compact alternative to steam engines, which had already been in used for over a hundred years. The steam engine had made the Industrial Revolution possible, but it was inefficient, bulky, and tiresome to operate.The answer was the internal combustion engine, with diesel-powered engines being one alternative, and petrol-powered engines being the other. Diesel engines eventually won favor over petrol engines for a number of reasons, but mainly because they were considerably more efficient and safer.
German inventor and mechanical engineer Rudolf Diesel is widely regarded as the founding father of diesel engines.
Today, diesel-powered engines are quite literally the beating heart of modern civilization. The taxis and buses that transport people around cities are powered almost exclusively by diesel engines; the ships that carry goods around the world are powered by diesel engines; the jeeps, trucks and tanks that form the backbone of the world’s militaries are all powered by diesel engines too.

Petrol and Diesel Engine how both are different 

The diesel and petrol engines that are used in most cars are highly similar. In essence, they are internal combustion engines that work using a two or four-stroke cycle. In an internal combustion engine, the power cycle is made up of four phases: intake, compression, power and exhaust.
In the intake phase, air is drawn into cylinder through the opening intake valve. In the compression phase, the intake valve closes and air is compressed with fuel. At this point, the mixture of fuel and air is ignited to cause an explosion. It is this explosion that causes the piston to downwards and drive the crankshaft to produce motion. This is the power phase. The final phase is exhaust where the spent air-fuel mixture is expelled out of the cylinder through the opening exhaust valve so that a new cycle may begin.

Petrol Engine 
Petrol working cycle

The main difference between petrol and diesel cycle is that petrol engine uses otto cycle to carry out uts cycle. In petrol engine spark plugs are used to ignite the fuel.

Diesel engine
Diesel working cycle 

Diesel engine uses the Diesel cycle, and spark plugs are absent,, the combustion process instantaneously begins when the air-fuel mixture temperature reaches the ignition temperature of the diesel fuel.

Therefore in diesel engines, air in cylinder would be very heavily compressed, typically to around 14 to 23 times its original volume. In petrol engines, the compression ratio is generally much lower, because they rely more on the spark plug to begin the power phase. The compression ratio is petrol engines is typically around only around 7 to 10, with high performance vehicles having higher compression ratios of up to 13.High compression ratios are desirable because it results in higher thermal efficiency. This also explains why diesel engines are considerabluy more efficient than petrol engines. In fact, diesel engines have the highest thermal efficiency of any internal combustion engine.

Pros n Cons 

Being very efficient, what other advantages do diesel engines have over their petrol counterparts? And what disadvantages do diesel engines have? Let’s take a look at some of the important ones briefly here.
  1. Not only are diesel engines more efficient, diesel fuel is also cheaper to purchase. At the time of writing, diesel fuel is around 40% cheaper per liter than petrol. This means diesel vehicles will be cheaper to run, which also explains why buses and most taxis have diesel engines.
  2. Because diesel engines are so efficient, cars can get incredible mileage out of them. It is not uncommon for passengers cars with modest 50 liter fuel tanks to be able to travel over 1000km on a single tank. This means more time spent traveling and less time spent refueling.
  3. To withstand the high compression of gases within the cylinders, diesel engines are built to be extremely hardy and will generally last longer than their petrol counterparts. They can also go longer between maintenance.
  4. Diesel engines can be made to run on alternative and renewable fuels like biodiesel with little or no modifications. Biodiesel generally refers to used vegetable oil that has been used for cooking, and is then recycled and treated so that it can be used to power diesel cars.
  1. Diesel engines need to be built stronger to withstand the high compression of gases, as a result, they usually cost more to manufacture. Consequently, diesel cars can sometimes cost more than their petrol equivalent. This depends heavily on the manufacturer.
  2. Diesel engines produce a distinctive knocking sound that is referred to as diesel clatter. This sound is the result of the sudden ignition of fuel which causes a pressure wave. It makes diesel engines sound less refined and noisier.
  3. Diesel engines are heavier and less eager to rev than petrol engines, which makes them undesirable in sports cars. This also makes diesel powered cars less peppy and engaging to drive.

Following are the points that conclude the high torque and efficiency of diesel engine.

Diesel engine uses simple mechanism for combustion unlike in petrol engine. Removal of ignition system not only makes the mechanism simpler but also reduces the risk of improper combustion due to damage in ignition system. In short burning of the fuel is easy and always accessible. This results in higher efficiency diesel engine.
Diesel fuel is a heavier hydrocarbon in which carbon and hydrogen are strongly bonded with each other. And when energy is supplied in form of heat it gets explode releasing much higher energy then gasoline. In short diesel fuel has higher energy density then gasoline which results in huge explosion.
One more factor for higher efficiency in diesel engine is its property of lubrication. Although all fuel has property of lubrication but diesel fuel has much higher lubrication then petrol fuel.
The compression ratio is much higher in diesel as compared to gasoline because in diesel engine air is alone compressed inside the cylinder and it’s a known fact that gas easily compresses then liquid. This is not so in gasoline engine because air-fuel mixture is compressed inside the cylinder. This higher compression gives higher heat and simultaneously higher torque.
We can’t use Carnot cycle to get 100% efficiency but can use its principle to attain maximize efficiency. In diesel engine heat is added at constant pressure which results in higher utilization of heat energy to get maximize work output.

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Diesel Engine vs Petrol Engine. Why diesel is more efficient?

Hello Everyone welcome to my new blog  While buying a new car everyone gets confused between the Diesel and Petrol motor. Will clear all y...