FSAE Intake and Exhaust Systems
Final Design Report:
November 29, 2001
Table of Contents
D) Morphological Chart
E) System Architecture
F) Function Analysis
When dealing with the internal combustion engine it is important to note that the engine itself is extremely inefficient in its use of the energy added into the system. Some of theses losses are innate in the combustion engine while others are limited to manufacturing trade offs. One such area that can greatly improve the characteristic of an internal combustion engine lies in the tuning of the intake and exhaust manifolds. Considerable increases in the volumetric efficiency of the engine and power output can be achieved by optimizing the design of these components. By focusing in on manifold design and options currently available the complexity of this topic becomes evident. A thorough and carefully thought out design must be employed to insure the best possible performance of the engine.
Design of an intake and exhaust system for a prototype racecar, scheduled for mass production at minimal cost for the weekend autocross racer. Intake and exhaust system must provide maximum horsepower and torque across the power band, provided a restricted intake and decibel limits placed on the design.
On the intake side of the engine air and fuel are mixed together and their flow to the cylinders is regulated. Several different components must work together for the intake system to perform these tasks successfully. The main components of the intake manifold include the air filter; throttle body, plenum, fuel injectors, and runners. The air filter removes impurities in the air so it will not hinder the combustion process. The throttle body provides the user with a means to manage the flow of air into the engine itself, increasing the opening to supply more air. The plenum serves as a reservoir for the incoming air to be pulled from when each cylinder requires a charge of air. Fuel can be regulated and mixed with the air through a number of methods from carburation to electronic fuel injection into the port. Then finally the air flows through the runners and into the engine where it is combusted. Choices exist for each system when tuning your intake, and this is where the complications begin.
Intake manifolds use a wide variety of shapes and sizes to take advantage of every possible combination of all the various components to maximize the needs of a particular engine. In doing so it is important to understand a few qualities that are desired in an intake system. First of all a smooth laminar flow is wanted in areas of the intake where no fuel is present. This will minimize the losses due to wall friction and bends in the manifold system. Yet, when fuel is introduced into the system it is necessary for the flow to become turbulent. This will increase the flow velocity at the same time increasing the atomization of the fuel air mixture to create a proper burn once in the combustion chamber. Variations in
the lengths and bends can be seen in Figure, where four types of plenums are illustrated.
The Main goal behind tuning an intake manifold is to increase the volumetric efficiency of the engine. The volumetric efficiency itself is a miss leading term, in that it is actually a measure of the mass flow of air into the engine. The volumetric efficiency takes into account the losses throughout the system from the air filter to the intake valves themselves. With the right kind of components the efficiency can actually be greater than 100% due to the addition of a super charger or turbo charger which increases the density of the air charge going into the engine. Volumetric efficiency itself is not a constant as the values vary for various engine speeds and pipe lengths as illustrated by the jaguar D-type engine in Figure. This
figure shows that at a higher rpm the longer pipes will actually hinder the efficiency of the engine leading to a poor performance curve. This phenomenon has led to the development of a new intake technology that involves manifold folding. Here the intake is able to vary its length based on engine rpm, by doing so the power band is maintained and an increase in the overall performance of the engine is gained. In order to create more horsepower the runners become shorter in length allowing the air a more direct path, and when torque is desired the pipe length is extended though the use of a valve to control the flow through a different runner configurations.
Another important consideration in the design of the intake manifold comes from the manner in which the fuel is introduced into the engine. Currently there are two main methods of fuel addition, Carburetion and fuel injection. Modern systems are increasingly moving to computer controlled fuel
injection systems due to a more efficient control mechanism. In a carburetor the fuel is introduced far upstream of the intake valves. This becomes inefficient because one must maintain a turbulent flow to create a good atomization of the fuel and air. Also in carbureted engines, the mixed fuel and air mixture can collect on the walls of the runners and plenum affecting the fuel to air mixture ratio. The benefit of this method to most old time car enthusiasts is that you can manually tune these types of fuel systems. Yet, for most people the current standard of multi-point fuel injection is much better. With this type of injection the fuel is introduced at the intake valve allowing easier flow upstream of the valves. Fuel injection systems also make it possible to change and monitor the fuel air ratio at a given rpm and change it accordingly based on feedback from the engine itself giving a cleaner more accurate burn.
On the other side of the engine another manifold is connected to assist with the exhausting of the noxious gasses left over from the combustion of the fuel and air. This exhaust manifold is equally important in its design to help achieve the proper breathing of the IC engine. The entire Exhaust system is comprised of the exhaust manifold, exhaust pipe, muffler and tailpipe. Once again the shape and size of the manifold become dependant on the objectives of a particular engine. For most engines a simple cast iron manifold is used due to cost and manufacturability. This technique is valid for most production manifolds, yet there has been a large push in recent years to “tune” the manifold and changing the name of it to a header. The push toward the design of a header has been motivated by an increase in demand for performance engines and also by trying to optimize the engine itself to comply with more stringent emissions standards. An optimal exhaust system creates as little backpressure as possible. This enhances flow through the system and prevents exhaust gases from flowing back into the cylinder creating backflow. To reduce backpressure a large free flowing muffler is desired as well as using a piping system with minimal bends thus eliminating all possible sources of loss.
The most important limitation in the design of the intake system is the restrictor constraint enforced by the rules of the FSAE competition. The restrictor must be placed after the throttle body, before the engine and can be no larger than 20 mm in diameter. This will severely limit airflow at full throttle so the rest of the intake system must be designed to be as effective as possible.
Ambient air first enters the intake system through the air filter. The air filter is necessary because small bits of debris can ruin the tight tolerances inside the engine. The filter should effectively and efficiently remove intolerable particles from the entering air stream while creating as little pressure drop as possible. Many years of research and development have been put into air filter design so this component will be vendor supplied.
The next component is the throttle, which comes between the filter and the restrictor. The throttle should be larger in diameter than the restrictor to prevent unnecessary pressure loss but the advantages of increasing the throttle size will die off after a point. The size of the throttle and the angle of reduction to the restrictor can both be varied and both will affect the pressure loss in that area of the intake system. A flow rate through the system must be used in order to calculate the losses due to the throttle and the conical reducer to the restrictor. For the calculation (4112.01) a volumetric efficiency of 65% was chosen for a rough approximation of the airflow at full throttle. The actual VE has not been measured but this is a reasonable estimate that can be used to show the trends of varying other parameters. While the head loss values may not be correct, the relationship between pressure loss and the geometric parameters can be shown and used to determine the best design. Calculation 4112.01 shows that the head loss decreases sharply as the throttle diameter increase from 20 mm to about 35 mm. Increasing the diameter above about 50 mm has no appreciable effect on the head loss. This suggests an optimum throttle size of about 40 mm. The actual throttle will also be vendor supplied, so the exact diameter will depend on what is commercially available, but 40 mm is the target diameter for this component. A throttle diameter of 40 mm and a reducing angle of 12 degrees yield a length of 9.5 cm for the reducing section (calc. 4111.02).
After the restrictor the air then flows into the plenum. Anytime a fluid exits a pipe into large volume there will be an associated pressure loss. This loss is proportional to the velocity of the fluid exiting the pipe. The air will have a very high velocity as it passes through the restrictor so it is advantageous to slow down the air before it enters the plenum. This will be accomplished by using a
conical diffuser between the restrictor and the plenum. Both the angle of expansion and the final diameter of the diffuser will affect the losses in this section. Calculation 4112.03 shows that the decrease in losses starts to level off when the exit diameter exceeds 40 mm and reaches an absolute minimum at approximately 60 mm. Another approach to sizing the diffuser is to maximize the pressure recovery. Figure 5 in calculation 4112.03 shows that a maximum pressure recovery can be achieved by using an area ratio of 3. This corresponds to an exit diameter of 60 mm, which is consistent with the pressure loss calculations. An area ratio of 3 and expansion angle of 10 degrees yield a length of 22.9 cm for the conical diffuser (calc. 4111.02).
The plenum acts to smooth out turbulence in the flow and assure that each runner “sees” the same flow area. Through correspondence with Tachih Chou, a technical specialist at Ford, it was decided that a plenum volume of twice the displacement of the engine. The plenum was designed to be close to this volume, but due to its complex geometry, the exact volume of the plenum has not yet been calculated. The design may change slightly once the volume is calculated.
With the rapid increase in engine technology modern manifold tuning is taking on a new face. Gone are the days of a mechanic tweaking a car by hand and making slight improvements to enhance the overall performance of the engine itself. Modern tuning is now at the levels where computers control and change engine configurations on the fly, much faster and more accurately than any mechanic ever could. More and more computers are being used to increase efficiency and even diagnose problems before they arise.
List of Drawings:
List of Calculations:
Calc # Title Rev. Author Date 4111.01 Runner Geometry 0 Anders 11/27/2001 4111.02 Restrictor Geometry 0 Anders 11/27/2001 4112.01 Optimum Throttle Size 0 Anders 11/14/2001 4112.03 Expansion Losses 0 Anders 11/28/2001 4121.01 Exhaust Pipe Length Calculations 0 Jason 11/12/2001