Talk of a huge Caterpillar 777 carrying a hundred tonnes, some heavy earth digging machinery by Komatsu or a muscular SUV grinding down the miles, they have one thing in common. Diesel engines. They were among the first revolutions in the automobile market, something which pushed people to change their minds about buying cars and consider something else over gasoline, with its distinct advantages over it.
Till very recently, diesels were the preferred choice for two vastly different segments of buyers; the ones, who are more family-oriented, went for long highway trips and the touring and off-road enthusiasts with their SUVs, due to their heavy duty construction, ability to do the toughest of tasks and fuel-efficient nature. Apart from these, diesel engines had and continue to have a monopoly in the market for freight trucks, heavy machinery and locomotive engines. But in the domestic segment diesel engines has taken a steep drop in terms of sales and that has been the case for most manufacturers. Some of them have even gone to the extent of stopping the production of some of their most reliable diesel engines.
In this article, we take a look at the introduction of diesel engines, their difference with other internal combustion engines (ICE), categories of them, a brief about how they work and the factors which have made them the “not-so-popular” choice anymore.
The Diesel engine was invented and named after its Canadian inventor, Rudolf Diesel, in the 1890s. A type of internal combustion engine with a different design and functioning, it burns fuel due to heat produced from compressed air in the cylinders of the engine, unlike other engines which burn due to heat produced from an ignition spark. Diesel engines fundamentally were based on the Carnot cycle, which was the first method to be used while making them. After numerous trial-and-errors and multiple patents later by several European nations, Diesel made the first prototype diesel engine in 1893. After two more prototypes, addition of a supercharger, several tests and commencing of commercial production, Diesel was a millionaire.
Over the 1900s and 1910s, diesel engines were used in ships and submarines of European countries. One of the early commercially successful diesel engines were made by the German manufacturer, MAN AG. They were the pioneers of the trunk piston engine, the two stroke diesels and the dual piston two-stroke engines. Over the 20th century. diesel engines have continued to develop and technological upgrades for better performance, fuel efficiency, smoother workflow of engine components have continued. Inter-coolers were introduced in the 1905 with the first indirect injection in 1910. Indirect fuel injection (FI) systems were introduced soon, followed by direct FI systems in trucks and locomotives in 1920s.
The first inline injection pump was made by Bosch in 1927 and the first diesel car in 1929. In 1933. the most successful aviation diesel engine named, Jumo 205 was introduced. The 1950s saw, air-cooled engines take over. In 1976, Volkswagen Golf became the first sedan to use a diesel. CRDi (Common Rail Direct fuel injection) designs were taken up for development and in 1997, the Fiat JTD became the first mass-produced car wit this design. Electronic fuel injection pumps were designed by Bosche and soon were a common feature in the cars of Fiat, BMW, VW and Audi. 2010 marked the introduction of VVT or Variable Valve Timing in a passenger diesel car by Mitsubishi.
Are diesel no more the “preferred choice”?
Over the years, diesel engines have built a reputation of being the most hardworking, efficient workhorses in the automotive and heavy machinery industry. But over the years, this perception has changed somewhat. Diesel cars which used to be almost half of the passenger car market in Europe and peaked at the end of last decade has started falling and stands at 36 percent in 2018. The first major reason for decline in diesel car sales is definitely the fact that diesel is a more polluting fuel. Although, unlike gasoline it produces less CO2 but has many other atmospheric pollutants like NOx( Nitrogen Oxides) and PM 2.5 (Diesel particulate matter).
These significantly reduced air quality but most countries accepted that as diesel engines had a reduced carbon footprint, which was the primary aim of many countries in environmental summits and international agreements. But huge pollution levels in cities due to increased levels of nitrogen and particulate matter, held diesel as the culprit. Moreover, the international public health and welfare body, WHO, announced diesel as a carcinogenic material, in a 2012 report. But this little spark changed into a wildfire with the 2015 Volkswagen scandal. Europe had emission norms for controlling nitrogen but these tests were easily fooled. Volkswagen, the world’s largest automobile manufacturer manufactured cars which used to fool these emission tests. The cars were equipped with special sensors, which allowed the car to understand the difference between real-time driving and testing and produce significantly lower emissions. This scandal led Volkswagen to publicly apologize and pay fines worth $35 million.
The second reason is somewhat an offshoot of this, i.e. increase in sales of electric and hybrid cars. With the Volkswagen scandal, top European nations like UK and France announced bans on sale of internal combustion engines. Automakers like Volvo and Nissan and most surprisingly announced phasing out of diesel engines and investing heavily towards electric and hybrid car tech. Electric cars takes one of diesel engine’s primary characteristics i.e. high torque output at low rpm to the next level. Electric motors can pump out peak torque at any rpm. Apart from that, they are much more cleaner and quieter than diesel engines too. Electrification of public transit systems like trains, metro rails, buses and taxi services have also grown, which has led to reduction of shares in segments, where diesel used to have a monopolistic hold.
But days of diesel are far from over. In most countries including USA, heavy machinery and engineering equipment, freight trucks, digging and hauling vehicles, equipment in mining, agricultural and construction industry and propulsion systems of marine crafts (ships, speedboats and submarines) are still run predominantly on diesel. Because, diesel still is much cheaper than electric power and running these huge systems will either require lots of charging stations or a large number of batteries which can keep them running for long distances, none of which are cost-effective.
How are they different from other ICEs ?
Diesel engines are fundamentally different from other internal combustion engines in the way the combustion occurs. While others use spark-ignition technology, diesel engines employ compression-ignition for combustion of fuel, more on which, we will look later in the article.
Apart from these , diesel engines are more fuel efficient, deliver a lot more torque making them suitable for heavy-duty work and are cheaper than their gasoline counterparts. Due to the very high expansion ratios and high AFR (Air-to-fuel ratio), makes diesel engine deliver the highest effective efficiency figures and thus they consume less fuel. In the realistic scenario, an average diesel car has an efficiency figure of around 40-43%. This figure is around 45% for large diesel trucks and buses and well above 50% for large and slow two-stroke diesel engines of ships,submarines and other watercrafts. This massive advantage of diesel engines, make them preferable over long distances and the difference is very evident when the vehicle is moderately loaded.
Another major advantage of diesel engines is that they can run on a wide variety of fuels. Petrol, LPG and gas turbine engines run on their namesake fuels. Diesel, although has primarily been obtained from fractional distillation of petroleum, can be made from other sources too. These are synthetic fuels, sourced from natural sources of fatty acids and and these are collectively called as Biodiesel.
Diesel is not just more efficient but also cheaper and has higher energy density, i.e. more energy is produced per unit mass of fuel burnt.
Diesel engines have much higher operating temperatures in their cylinders than petrol ones, thus allowing the use of less volatile fluids. This makes them suitable for many types of fuels, assured they are of a proper viscosity.
A diesel engine is the most suitable engine choice for vehicles which do heavy-duty work like carrying goods over long distances or digging and hauling up large amounts of earth. These type of tasks require engines which produces high amounts of torque, something a gasoline engine is not adept at. The major reason behind this is the fuel and high compression ratio; diesel has a lower calorific value than petrol and also is more energy dense. Therefore, they burn cleanly and the pistons are pushed with more energy, thus increasing the torque generated through the crankshaft. The compression ratio is basically the ratio of minimum and maximum volumes of a cylinder. Now, because compression ratios are much higher in diesel engines, the piston moves through a longer distance down a cylinder with larger volume, i.e. it has a higher stroke value. Higher stroke value transcends into more torque, as the longer lever of the piston drives the crankshaft with a stronger force. Also, diesel engines are low-revving; thus they need to have a high torque and do more work in smaller amount of time to generate substantial power.
They also allow only air to enter the combustion chamber before it gets compressed and reach high temperatures. This removes the risk of pre-ignition, which can cause engine damage, as the fuel is only allowed after their is substantial amount of pressure inside the chamber. Although, the fuel injection time should be ensured to be appropriate.
There are a few disadvantages of diesel engines too. Despite delivering more torque, diesel engines essentially have lower engine revolution speeds, which restricts the power they generate. Also, due to extreme high pressures in the combustion chambers, the engine components are much sturdier and heavier to resist the inertial forces. This makes diesel engines heavier. Some diesel engines, especially the older ones need heat to start, as they lack an ignition spark plug and thus can be difficult to start in cold temperatures. Although, this is solved in most cars with the modern CRDi injection designs which can start at temperatures of zero degrees or by the presence of a heating aid called glowplug. Larger engines, like those in trucks use flame-start.
Although, diesel burns cleanly compared to gasoline with significantly lesser CO2 emissions, due to the ability to “lean” burn or use more air and less fuel to produce the same amount of energy, they have other emissions which are really harmful. Diesel engines emit carbon monoxide (CO), nitrogen oxides and particulate matter due to incomplete combustion and dissociation reactions. Diesel exhaust is a Group 1 Carcinogen and can cause lung and bladder cancer.
How does a diesel engine work?
In petrol and LPG engines, there is an ignition spark plug on the combustion chamber which ignites the spark and burns the mixture of air and fuel. This combustion creates a pressure inside the chamber, which forces a piston to move downward and turn a crankshaft. This leads to one single revolution. The number of revolutions per minute(rpm) is the engine speed.
In diesel engines, only air is allowed inside the combustion chamber at first. The air is then compressed under very high compression ratios, which increase the temperature of the air to well above the ignition temperatures of the fuel. The fuel is let in through the fuel injector inside the combustion chamber. The fuel droplets turn into vapor and then get ignited, due to the heat produced from compression and all of this occurs at a constant substantial pressure. There is slight delay in the ignition of a diesel engine compared to a petrol one, caused due to the time to taken in vaporization of the diesel particles. This is the reason for the distinct noise of diesel engines called “diesel clatter”. After the fuel is burnt, the gases in the chamber expand and that forces the piston to drive down and turn the crankshaft.
It should be kept in mind, that fuel quality and injection type and methods also play a crucial role. Fuels which do not have cause delay in ignition and willingly burn, i.e. fuels with high cetane rating can be uniformly used well by all kinds of injection systems. For comparatively inferior quality diesel fuel or even biodiesel, certain pump designs like inline mechanical injection ones work. Engines with indirect fuel injection work better on fuels with higher ignition delay because they improve vaporization and therefore combustion.
Most diesel engines today are equipped with direct fuel injection and turbocharging, some of them with supercharging too. Turbocharging or supercharging are methods which compensate for the lack of power output in diesel engines. A turbocharger pushes more amount of compressed air in the engine’s cylinder, thus improving the efficiency of fuel burning and increase power output. So a turbo-charged engine is always more powerful than a naturally aspirated one. Superchargers have the similar function but they are mechanically powered by engine’s crankshaft rotations while turbochargers are powered by turbine motors which use the exhaust gas.
Different categories of diesel engines
Diesel engines can be categorized into different types based on their power output, cylinder position, stroke cycles and engine speeds.
Basedon power output
They are of three major sizes, based on power output. Small diesel engines produce within the range of 252 hp (188 kw) and are the most common type. They are used in all passenger cars, light pickup trucks, lifestyle vehicles and small machinery. They are generally inline-4 or inline-6, direct fuel injection engines; some come with turbochargers and inter-coolers too. Medium size engines are mostly used buy trucks and other heavy machinery like road rollers and earth diggers. They generate power output from 250-1000 hp and are generally turbocharged engines with , inline-6, V-8, V-10 or V-12 configurations. Large engines are exclusively produced for watercrafts (shifts and submarines), locomotive and electricity generation. These produce power in excess of 1000 horses.
Based on cylinder position
Engines come with various types of cylinder configuration, based on their number and arrangement methods. If the cylinders are arranged next to one another, in a straight line it is an inline arrangement. This is the most popular type as they are simple and smooth. But you will mostly find them in 4 or 6- cylinder configurations. The only disadvantage is they take up space, so fitting more than 6-cylinders is not viable.
Next is the V-type engines, where the cylinders arranged in a V-shape on two sides of the engine. They generally generate more torque,take up less space but the cylinders need to be placed at the correct angles, for managing the firing order. For a V6 it is around 60 degrees, a V8 is best suited at 90 degrees and a V10 is comfortable at angles of 72 degrees. A V12 is slightly different, it is basically two inline-6 cylinder sets meeting together at the crankshaft. No doubt, they are smoother! V-type engine designs have two cylinder heads which makes the construction complex and more costly.
The final type is the flat or boxer type. The cylinders are place at an angle of 180 degrees to each other. They help in better handling due to their low center of gravity and have been the oldest type of engines.
Based on engine speeds
Three major types here as well. High , medium and low speed engines. High-speed engines are those with engine speeds of over 1000 rpm. These are found in most cars, trucks, buses and small machinery. They are also used as engines for powering generators. They generally have direct fuel injection and power ranging from 5 hp to 6000 hp.
Medium speed engines are used in equipment like small and medium sized boats, large pumps and compressors, propulsion systems of ships. They use high viscosity fuels with direct fuel injection methods and are very efficient, well above 45 percent. They have speeds of 300- 1000 rpm and can generate power above 20000 hp. They are cost-effective compared to low speeds engines.
Low speed engines are designed to run with speeds of less than speeds of 300 rpm. These are extremely large and mostly propel ships or trains. They use a highly viscous and heavy fuel and provide efficiencies up to 55 percent. Two common types of these are, a cross-piston two stroke and a trunk piston four stroke. The former helps in propulsion systems while the latter, powers the electric generators in ships.
Based on stroke cycle
They are of two types, two stroke and four stroke cycles. Four stroke engines are more common nowadays and are largely found in all automobiles and machinery. Two strokes have gradually faded from the consumer market.
In a four -stroke engine, there are intake and exhaust valves and a fuel-injection nozzle. The first stroke involves the intake of air in the chamber followed by the second stroke where the air is compressed and the piston head reaches the TDC. In the third stroke, the fuel is injected from the injector nozzle and the combustion of the fuel pushes the piston which drives the crankshaft to produce power, i.e. the fourth stroke. A two stroke engine eliminates the use of an intake valve atleast, if not the exhaust valve. Filling the cylinder with air and compression is one stroke, while combustion and power generation is the second stroke. Intake is done by intake ports instead of, valves and exhaust through exhaust ports or valves. Due to the lack of valves, two stroke engines need some sort of pressure pumping to push in air and release the gases. This is called scavenging.