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Automotive Engines By: Andrew Chasin
Abstract Automotive engines are very complex machines that work in order to create power for the automobile. This is done through engine strokes and combustion, which are the essentials in knowing how the engine actually works. Although, not all engines are the same, there are many different engines, some of which aren’t covered here because of their complexity. In this report, one will learn how the engine works, how the power is created and transferred, the differences of various engines, how superchargers and turbochargers increase power and about engine performance. Some concepts of thermodynamics are used in order to aid in the description of engine strokes. All in all, even without any knowledge of automobiles, one can come to understand how engines work through reading this report.
Table of Contents
Literature Survey Narrative
List of Figures
Turbochargers & Superchargers
Intakes & Miscellaneous Parts
Recommendation for Further Study
Literature Survey Narrative I decided to choose automotive engines once I realized vehicle dynamics may have been too difficult. I had no experience with automotive engines prior to writing this report nor did I have any knowledge of engines. It took me a while to get the ball rolling on this report due to the fact that I didn’t know anything. I decided to go to my Formula SAE advisor who then explained how it all worked with diagrams and thermodynamic concepts. I then looked further into these topics, which are the subheadings, and was able to delve deeper into the topics to master my overarching topic. The book “Introduction to Internal Combustion Engines” helped bring everything together considering there are so many different parts of the engine that do their own jobs. Once I learned engine strokes, the rest of the engine basically helps aid in that process which made it less difficult for me.
List of Figures
Figure 1.1 Figure 1.2
Engine Strokes There are several types of engines, two of them are 4 stroke and a 2 stroke engines. Most other engines follow the same format, having multiple strokes in order to create power for the car. The four strokes in an engine are intake, compression, expansion or “power stroke”, exhaust. This 4 stroke procedure is usually seen in V-Type engines, not in diesel engines. A V-Type engine requires a spark plug above the combustion chamber to ignite the gasoline when a diesel engine does not. The first stroke in an engine cycle is the intake. This is where the air/fuel mixture travels through the intake valve into the combustion chamber. The second stroke is compression, where the piston is forced upward by flywheel in the crankcase. In addition to this movement, the camshaft is located near the fly wheel and it’s what converts rotational motion into linear oscillating motion which is what moves the piston up and down. The third stroke is called expansion or the power stroke. This is where most of the power is created, the spark plug ignites the compressed fuel and drives the piston downward. The exhaust stroke is the final stroke and comes at the end of the power stroke. With the downward motion of the piston, the exhaust valve opens up and while the piston goes back up compressed fuel/air mixture goes out. This results in the cycle starting back over. Similar to this is the 2 stroke engine, it does all 4 steps much like the 4 stroke but only in 2 steps. 2 stroke engines are usually more powerful than 4 stroke engines. This is because the 2 stroke engines have a power stroke every 2nd stroke and complete their cycle faster than the 4 stroke engine. The first stroke in the 2 stroke engine is the intake and power. This is when the intake valve is opened by the upward force of the piston. Since the piston is going upward, the fuel/air mixture from the previous cycle explodes in the combustion chamber in order to create power. Then the piston is driven downwards, while this happens the waste from the explosion goes out as the exhaust part of the cycle and the new fuel/air mixture is compressed. It’s easy to tell that the 2 stroke is more powerful considering the cycle happens much quicker.
Engine Types There are numerous types of engines, such as V4-V8, diesel, HEMI, inline, etc. The most common engine type seen throughout everyday life is the V4 or four cylinder engine. Each “cylinder” has their own 4 or 2 strokes stated in the previous section. The more cylinders in the engine means the more cycles the engine can go through and the more power strokes will occur resulting in more power (Stone, 1985). The difference between the V-type engines are the amount of cylinders it has. For example, a V4 engine will have 4 cylinders opposed to a V8 engine having 8 cylinders. The V4-V8 engines all have spark plugs which ignites the fuel. Meanwhile, diesel engines don’t have a spark plug, the diesel fuel has a different chemical makeup which enables it to explode at a specific temperature and pressure. Non-diesel engines use what’s known as petrol or gasoline which explodes with the help of the spark plug. Non-diesel engines can “time their engine” to begin accelerating exactly when the spark plug explodes the gasoline in order to gain the most amount of power at the start. In most cars, the amount of liters in the engine are usually advertised. There is a specific ratio as to what amount of air and gasoline are in the mixture that enters through the intake valve. The usual ratio, air to petrol, is 15 to 1 (InfoSpace LLC, 2015). The liters are referring to how much air is let into all of the pistons. For example, if an engine is said to have 5.4 liters, that means the engine is able to let 5.4 liters of air into all four pistons after 2 revolutions of the crankshaft. Obviously, the more air let into the engine means the more power will be created, so the more liters in the engine means that the engine is generally more powerful than those with less. There are also many different types of engines, one of which being V shaped. V shaped engines have usually 6 cylinders with 3 on each side and one opposite the other at an angle of about 120 degrees. As one piston on one side goes up the other has the opposite motion.
Combustion Combustion is what creates the power from the power stroke. There’s a lot more to it than just a timed explosion, there are processes such as the otto cycle and the diesel cycle that occur that are crucial in engines. The otto cycle uses the 4 stroke engine cycle in order to show how pressure and volume increase and decrease in order to create power (Hall, 2015). As one can see in the figure, the intake stroke increases volume, then the compression stroke
decreases the volume by increasing the amount of pressure. This can be more understood as Boyle’s law, where pressure and volume are inversely proportional. The combustion then happens rather quickly while the temperature and pressure increase. Thereafter, the gas expands causing an adiabatic process where the temperature and pressure will decrease without the loss of heat; energy is only transferred as work (Thermal Science, 2011). The diesel cycle differs from the Otto cycle due to the fact that the diesel cycle doesn’t require a spark plug.
Engine Performance Horsepower and torque are two things that are often advertised in new car commercials or car ads. Horsepower is supposed to gauge how powerful the car is and how fast it can accelerate. This relates to engines because the more powerful the engine is the more horsepower it will put out. For example, generally a V8 engine will put out more horsepower than a 4 cylinder engine. Horsepower increases as the number of revolutions per minute increase as seen in
figure 2.1.Horsepower is calculated by the equation HP =
where HP is horsepower, RPM is
revolutions per minute, and T is torque. (Simple Motors, 2015). This equation can also be used to convert horsepower to torque and torque to horsepower. When racing, one wants to keep the engine near the peak horsepower in order to maximize acceleration. Designers of high end super cars are often faced with the
challenge of increasing the power to weight ratio because the higher the ratio, the faster the car. Torque is another aspect of engine performance that’s talked about a lot, and it’s defined as the rotational motion from the internal combustion engine (Craig, 2014). A car engine creates torque by the rotational motion of the crankshaft previously stated. As stated previously, the crankshaft turns rotational motion into linear oscillating motion of the piston through the connecting rod. There are various types of torque in an automobile, although engine torque is what gets everything moving.
Turbochargers & Superchargers Turbochargers are placed into cars in order to increase horsepower and to increase the efficiency of the engine. Also, turbochargers do not significantly increase weight so it makes sense as to why turbochargers are found in most sports cars. In short, a turbocharger will compress the air prior to entering the cylinder through the intake valve, resulting in more air in the cylinder which ultimately correlates to the car receiving more power (InfoSpace LLC, 2015). This may seem like a no brainer to add this to one’s car if they want to increase power.
Although, there is a drawback to adding a turbocharger to a car. The turbine in the turbo spins because of the exhaust flow which increases back pressure during the exhaust stroke in a cylinder. This means the engine must work harder in order to get the fumes through the exhaust valve which causes a slight decrease in power from all cylinders. Another downside of having a turbocharger is a concept known as “turbo lag”. This refers to the delay in power because the turbine needs to start spinning once the exhaust fumes start flowing and once it does the car
lunges forward. In essence, turbochargers are mainly for that quick extra boost in acceleration or horsepower, they don’t aid in increasing top speeds. Turbochargers and superchargers accomplish the same goal of increasing power in different ways. A supercharger uses fuel because it’s connected to the crankshaft by a belt, although there is no lag.
Intakes & Miscellaneous Engine Parts The intake of a car does exactly what it sounds like, it sucks in air. There are 3 categories of intakes, cold air, short pipe, and ram air. Obviously each type of intake has it’s ups and downs, although they all share a common goal of getting as much air in through the intake valve as possible. Different intakes may give you more power or even make you're engine sound better. All intakes generally want to take in cold air because cold air is less dense than hot air, therefore you can get more air in through the intake if it’s cold rather than hot. This is where the short pipe intake comes in. It has a high flow filter and is efficient at high RPMs, but with this it sacrifices inhaling hot air radiating from the
engine (Cars Direct, 2012). On the other hand, a cold air intake draws air away from the engine components so it’s not heated by the engine, making the air denser which means the engine will produce more power. A ram air intake is similar, but this intake has a special air collector that forces extra air into the cylinders when the car moves forward (Cars Direct, 2012). Another aspect of intakes is the amount of air is flowing in through it. The velocity and pressure that the air is flowing at does matter because of the inverse relationship of the two. This is seen in the dynamic pressure equation p = 1/2pv^2 which describes how the air moves through the valves (NASA, 2015). The air flow is often manipulated in race cars in order to decrease pressure or temperature depending on whether they’re looking for more acceleration or torque.
Conclusion When looking at an engine work, one must look at each piece individually in order to see how the entire engine works. Each piece does it’s own job, whether its converting rotational motion to linear oscillating motion, or opening the intake valve, all the engine parts work together to create power for the automobile. The main engine type seen in everyday driven automobiles is the 4 stroke engine because of its efficiency. The otto cycle is also discussed in order to help explain the four strokes and to see how power is made based off of pressure and volume. The intake is also crucial to an engine because the ratio of gasoline to air is essential in determining how much power will be produced. In essence, automobile engines may look complicated, although once one takes a closer look, it isn’t incredibly difficult to understand.
Recommendation For Further Study The first place to start for further study would be looking at either drivetrains or automatic versus manual transmissions. “The Automotive Transmission Book” by Robert Fischer would be a great place to start one’s research as it tells how the automatic transmission works and how the engine works along with it. Basically any other part of the car would be a great place to start considering one now sees how the engine creates power. One can now see how that power is transferred to other aspects of the car in order to create motion. Also, one can see what happens to the engine when the car is in park, neutral, etc. This will lead the reader to becoming more car say because they’ll be able to compare different components of the cars and see how they work together.
Bibliography Hall, N. (Ed.). (2015). Ideal Otto Cycle. Retrieved November 8, 2015, from https:// www.grc.nasa.gov/www/k-12/airplane/otto.html V.A., K. (2015, October 15). Adiabatic Conditions. Retrieved November 25, 2015, from http://www.thermopedia.com/content/290/ Stone, R. (1985). Introduction to Internal Combustion Engines. Basingstoke: Macmillan. "What does 2.4 liter mean in the context of an engine?" 23 July 2001. HowStuffWorks.com. 10 November 2015. Nunni, R. (n.d.). Chassis Weight and Dimensions. Retrieved November 8, 2015, from http://alison.hine.net/gpl/grehelp/tables.htm Calculations. (n.d.). Retrieved November 29, 2015, from http://simplemotor.com/ calculations/ Horsepower and Torque. (n.d.). Retrieved December 5, 2015, from http:// craig.backfire.ca/pages/autos/horsepower Cutler, C. (2015, February 3). Turbocharger vs. Supercharger - What's The Difference, And Which Is Better? Retrieved November 28, 2015, from http://www.boldmethod.com/ learn-to-fly/systems/turbocharger-vs-supercharger-differences-and-which-is-better/ Turbododge.com. (n.d.). Retrieved December 8, 2015, from http://www.turbododge.com/ forums/f4/f15/357413-cold-air-intake-installation-question-4.html **Sources for figures are included above**