FEE Engineering Students: Part I Formative Essay – Engine Efficiency
Since the birth of the first internal combustion engine, designed by Siegfried Marcus in 1864, motor engines have been continually evolving. Engines encountered in modern day road cars bear little resemblance to their ancestors. This evolution is due in no small part to the rising popularity of Formula One motorsport. The top four teams spend several hundred million pounds every year towards research and development. These innovations eventually benefit the road car industry and therefore all of us. We now enjoy cars which are lighter, faster, more fuel efficient and vastly more reliable than in the 1950s.
The basic principles of the internal combustion engine remain very much unchanged over the years. Fuel enters a combustion chamber (the ‘cylinder’) which is burnt, the resulting chemical and thermal energy being converted to kinetic energy through the pistons. The past decades have seen improvements in efficiency and performance for every engine component, ranging from changes to cylinder configuration, valve design, crankshaft design, injection systems and sophisticated electronic engine control systems.
F1 engines are extreme in every way. They are capable of handling 18,000 rpm and exhaust gases with temperatures of up to 800 degrees Celsius [2]. When F1 engines rev up to 18,000 rpm, the force on the pistons is estimated at nearly 9,000 times gravity (9000G) [5]. Manufacturing F1 engines and cars which are more economical and powerful, has contributed greatly to increased engine efficiency in road cars.
In the 1977 Formula One season, turbocharging was introduced. The technology proved to improve the speed of the car however it suffered from turbo lag. This is due to the engine revving too low so there is not enough fuel being pumped into the cylinder. In a modern formula one car, they use their Energy Recovery Flywheel to keep the turbo turning so there is no lag. Back in 1977, engines suffered from overheating and multiple failures due to unreliability and an increase in weight due to the turbos. Superchargers have also been used, however due to the excessive fuel consumption and difficulties with cooling; they have been overlooked by teams that want instant power without any disadvantages. A turbo is a device that increases the internal combustion engines efficiency and power by forcing extra air into the combustion chamber [6]. Less fuel can therefore be used to give the same power output as without the turbo [14] .
Lewis Hamilton’s title winning Mercedes engine does not use a conventional turbo like Ferrari and Renault. The turbo is made of two parts, a compressor and a turbine. The compressor is in front of the engine and the turbine is at the back. With the turbine at the back, it has a shorter inter-cooling route, therefore less turbo lag and since the cooler temperatures allow for smaller intercoolers, the car is more aerodynamic. Having the turbo at the back of the car also allows the gearbox to be closer to the middle for better weight distribution and therefore, better handling [14]. This allows for later braking and more speed through the corners. With the compressor at the front, this allows for a shorter distance the air needs to travel from entering the car just above the driver’s head. Intercooling is when the pressure of the engines air intake is increased, however so does the temperature. The hot exhaust gases being re-circulated into the turbo heats the intake air. The warmer the air, the less dense and therefore, the less oxygen enters the combustion chamber. This reduces efficiency and also power.
An intercooler cools the intake air. The very first turbocharged car was the Oldsmobile F85/Cutlass in 1962 that utilised a V8 and the first Diesel powered turbocharged car was the Mercedes Benz 300 SD introduced in 1978. Nowadays, most diesel engines are turbocharged to improve performance and efficiency, due to its poor combustion [15].
Carburettors are a device that blends air with fuel ready for combustion. They work on Bernoulli's principle. The faster air moves, the lower its static pressure, and the higher its dynamic pressure. The throttle controls the flow of amount of air that enters the combustion chamber, whilst the volume of fuel that enters stays constant. Multiple carburettors are used in order to fuel different chambers of the engine’s inlet manifold. The inlet manifold supplies the combustion chamber with the fuel and air mixture. It evenly distributes the fuel and air mixture to every intake port of the cylinder head of the combustion chamber.
Direct Injection has taken over from carburettor as it is more economical, as instead of limiting the amount of air entering the combustion chamber, it limits the amount of fuel, and therefore decreases fuel consumption. Direct fuel injection involves squirting fuel, in the form aerosolised fuel, mixed with the intake air into to the cylinder by a pressurised injector which is fixed to each intake valve rather than using one injector [7]. The air and fuel mixture is drawn into the combustion chamber by the retreating piston which creates an area of low pressure. The intake valve then shuts and the spark plug ignites the aerosolised fuel [8]. The 1955 Mercedes Benz 300SL was the first car to utilise fuel injection. They were fixed to the cylinder wall where the spark plugs were, which were in turn, mounted to the top of the cylinder head [13].
Variable displacement is an engine technology that allows the reduction of engine displacement. It is commonly used when the engine is only being used to 30% of its maximum power in order to increase efficiency and decrease emissions. By not pumping the air and fuel mixture to half of the engine’s cylinders, the amount of fuel used is considerably less. Since the throttle valve is nearly closed, the pressure at the top dead centre of the piston is very low. The low cylinder pressure does decrease fuel efficiency; however, it is superficial compared to the fuel saving when shutting down cylinders. It was first used in the Mitsubishi 1.4L 4G12 straight-4 engine, which Mitsubishi called Modulated Displacement [3].
Cylinder deactivation is controlled by a module called the engine control unit, ECU, that controls the actuators to improve engine efficiency. A desmodromic valve is a reciprocating engine valve that is positively closed by a cam and leverage system, rather than a conventional spring. The valves in an engine allow the exchange of the air and fuel mixture into the cylinder and exhaust gases to be leave. In an engine, valves are opened by a cam and closed by return spring. An engine using desmodromic valves has two cams and two actuators and close the valve without the need of a return spring. This allows the engine to spin at extremely high speed which a spring would not be able to handle as they would break due to metal fatigue. The valves allow a certain amount of fuel and air to enter the combustion chamber so it does not waste fuel. Fuel consumption therefore decreases. Desmodromic valves were first used in the 1954 Mercedes-Benz W196 in Formula One and they were also used in the 1955 Mercedes-Benz 300SLR.
Figure 2: A Desmodromic Valve
An actuator is a type of motor that is responsible for controlling or moving a mechanism. It relies on electrical energy to move or hydraulic fluid pressure, or pneumatic pressure to control the mechanism. They can be used to introduce a motion such as an electric motor or hydraulic cylinder. There are four types of actuators, there are hydraulic, electric, pneumatic and mechanical. They all have their advantages. The electric actuator is the cleanest as it does not require any oil.
Cylinder deactivation can be taken a step further by switching all off all of the cylinders in the engine when the car is stationary. This is also known as switching off the engine. Drivers in Europe spend at average 25% of their driving time at idle [6], and drivers in the United States of America waste approximately 3.9 billion gallons of petroleum per year. This does have its limitations especially when most fuel is consumed when either starting up an engine or increasing the revolutions of an engine.
Engine blocks house the cylinders responsible for powering a car. To improve efficiency, reducing the weight of the engine block decreases the weight over the front axle and pushes the weight distribution towards the rear of the car. This decreases the overall weight of the car and increases efficiency. Nowadays, engine blocks are being made out of an aluminium alloy as it is much lighter than the standard cast iron block, and they also do not rust which improves reliability. Unfortunately, aluminium engine blocks distort under a high amount of stress which can cause damage to the cylinder bores and lose their seals which can decrease engine performance. This is due to their low density. The density of cast iron is roughly 6,800 to 7,800 kg/m3 [21] whereas the density of aluminium alloy A356 is 2,700kg/m3 [20].
In a world where fossil fuel reserves are being depleted and an increase in production in greenhouse gases. The car manufacturers of today are looking to other means of energy to safeguard our planet. Electricity is the up and coming source of power which will lead to a cleaner way of commuting from one place to another. At first, cars used electric power as an extra boost when initially starting off, as electric motors produce maximum torque instantaneously, whereas petrol engines have to increase the amount of revolutions per second in order to have the necessary amount of torque to set off.
Figure 2: The Power and Torque outputs in an electric car Figure 3: The Power and Torque output in a petrol car
Battery powered cars could be the way forward. ‘Formula E’ is an electric subgroup of Formula One that uses electric power as propulsion. Electric vehicles produce less emissions than a conventional engine however, there is a need for a large amount of energy needed to power a car [12]. This means a lot more electrical energy is needed to produce the same amount of energy as a petrol or diesel engine. To combat the low economy of electrical energy, a lot of lithium ion batteries would be needed to store the energy. Lithium ion batteries are extremely heavy. Weighing 155kg, a SmartBatt combines 1,408 batteries with a power rating of 181 Wh/kg [11]. A Tesla Model S has 6,800 batteries producing 70kWh of power [10]. This adds up to an added weight of 748.6 Kg. This added weight plus the motors weight should make the car very heavy, however the electrical energy is instant, therefore the Tesla can reach sixty miles per hour in 5.2 seconds [10]. Electrical power can be recharged using renewable energy sources such as solar energy or wind energy, unlike Hydrogen gas. This proves that these two fuel replacements aren’t environmentally viable.
The Toyota Prius is a car famous for using a blend of electric power and engine power when moving. It stores the electricity in banks of batteries made out of nickel-metal hydride (NiMH), however since 2015, Toyota have started using lithium-ion batteries as they are lighter and last longer. NiMH batteries are larger and heavier than Li-ion batteries and although they can carry the same amount of power, lithium-ion batteries can be charged and discharged more rapidly. Lithium-Ion batteries don’t suffer from ‘memory effect’ which can cause a battery to decrease its capacity when being charged before it is fully empty.
Figure 4: A flowchart of electric and engine power in a Hybrid car
The Toyota Prius only has two batteries which contributes to its small range. This therefore requires a way of charging up throughout a journey through a thermal recovery system. The energy recovery system used in formula one has made its way to the motor car industry due to higher requirements in fuel economy. Since 2013, A kinetic energy recovery system (KERS) has been fitted to six London buses [4] in order to reduce their fuel consumption and emissions. For the Road Car industry, Jaguar have introduced this technology into their latest concept supercar. The Jaguar C-X75 uses a flywheel KERS developed by Williams Formula One team. It highlights the ever-growing need for a renewable source of energy to replace fuel.
Figure 5: A Flowchart of Energy Recovery System to the Engine
Hydrogen gas is readily available as it is the most abundant element in the universe, another positive for the use of hydrogen is that when burnt, it does not give off any harmful by-products, the only product from is water. when used in NASA’s spaceships, the burned hydrogen gas leaves behind clean drinking water for the astronauts. Hydrogen is non-toxic compared to petrol which is. Hydrogen is a very efficient fuel and when burnt, it produces up to three times as much energy as when petrol is burnt, this also means less hydrogen is needed to produce the same amount of energy as petrol, therefore it is much more economical and efficient as it will last longer [9]. Lastly, hydrogen is a renewable source of energy as it can be produced on demand from abundant elements.
There are some disadvantages to using hydrogen as a fuel in cars, it is very expensive as the process of obtaining pure hydrogen is very costly. It is not a viable source of fuel for some therefore it will take a long time until they have a cheaper way of obtaining it and using it in production cars [12]. Hydrogen gas needs to be stored in a tank which is also very dangerous in the event of a collision. Hydrogen is flammable and volatile so it is potentially deadly if lit. Hydrogen gas would also need to be pumped through the conventional pipelines which would be a problem as the would need to be replaced which would be extremely expensive. Cars would also need to be adapted to run on the gas which would cost car owners a lot of money. Hydrogen gas relies on non-renewable sources like coal, oil and natural gas to separate it from oxygen [9]. Which will not affect our carbon footprint so it would not be viable to replace fuel with hydrogen gas.