Intake

The intake system has a dominant effect on the torque curve and behavior of the powertrain system. All air used by the engine(s) for combustion must pass through a 20 mm or 19 mm opening for gasoline or E85 fueled vehicles respectively, commonly referred to as a 'restrictor'. The restrictor limits the theoretical power limit of the engine.

Intake Design

not sure if good order, WIP

Structure

Diagram of NA Intake System

Following the path the air follows as it enters the engine, the naturally aspirated intake system is made of the following components: the air filter, the throttle body, the restrictor, the plenum, the manifold, and the engine mounting. The air filter's sole responsibility is to stop water or particulates from entering the engine, these could cause engine damage or even failure. The throttle body allows for driver modulation of mass air flow into the engine, there-by controlling engine torque and speed. The plenum acts as a capacitor to smooth air flow into the cylinders and can be tuned to increase volumetric efficiency. The manifold consists of runners that channel air from the plenum into each cylinder.

Forced Induction Intake System Diagram

In forced induction applications, allowing a manifold between the restrictor and throttle body will limit the effects of the restriction. This is negated in FSAE by preventing a large volume (plenum/manifold, intercooler, etc.) prior to the throttle body (Rule IC.2.5.3). The order of system components defined by rules can be seen in the figure right and each component is discussed further down this page.

Packaging

I.C.1.2 Packaging Restrictions

Packaging of the intake manifold has minor if any impact on the performance of the engine. All intake packaging schemes can be categorized into the following: A top feed (or center feed), Side feed, and bottom feed. Although not true for every team, the choice is often based on manufacturing, serviceability and similar non-performance goals. Certain form factors lend themselves to intake manifold styles and vice-versa. Conical spline intakes are almost always found in a center feed configuration. Log style intake manifolds when used in a side feed configuration can be packaged very tight.

• http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.582.6654&rep=rep1&type=pdf referenced in plenum section

Intake Form Factors

Material Choice

The material used for the intake has a much lower impact on the performance than other tuning variables but warrants consideration. The major driving factors to consider are feasibility related: whether it is cost-effective, feasible to create the design the team has made, reliable enough to last through testing and competition, and similar. There are of course differences in the materials chosen that will be discussed here, but is important to understand the material choice is not expected to make a significant difference in dynamic event performance.

Common materials used by teams are below (Please add to this list if I've missed one)

Type	Material	Construstion	Notes
Metals	Aluminum	Welded sheet and tubing	May exchange heat with ambient air if not coated
Plastic	Nylon	Usually 3D printed	Nylon is one of the few 3D printable plastics that is gasoline resistant
	Nylon - Glass Fiber Reinforced	Usually 3D printed	Often outsourced, many institutions do not have in house capabilities to print this or won't want to
	Other	Off the shelf injection molded	Available 3rd party solutions
Composite	Carbon Fiber	Vacuum bagged, Infusion, Prepreg
	Fiberglass	Vacuum bagged, Infusion	Not as common as carbon fiber

Plenum

Because the engine takes in air in only one part of the combustion cycle, the airflow out of the intake system is inherently pulsed. The pulsing allows less mass airflow into the system than a smooth airflow would (for many reasons, the most basic of which being that the low pressure waves reduce the airflow more than the high pressure waves increase it).

One useful analogy is comparing the system to a circuit where the output current is pulsed. The restrictor acts as a resistor (hopefully obvious). A basic way to smooth out the circuit is to add a capacitor. In the real world, we do this by adding a volume of air after the restrictor known as a plenum.

The plenum in series with the restrictor creates a pseudo low pass filter that will help smooth out the pulsed air flow characteristics of the intake. Since the restrictor is a fixed size, the "circuit" is tuned by changing the capacity (capacitance hint hint) of the plenum. If the plenum is too small, the non-uniform nature of the intake will significantly decrease power output. If the plenum is too large, the throttle will no longer be able to predictably control the air flow.

The size of the plenum is tuned imperially with many FSAE papers available for comparison. The shape of the plenum can be assumed to be negligible, the dominant characteristics of the plenum shape will likely be your discharge/flow coefficients at the interface to the restrictor and runners.

The phrase dual plenum can refer to two different designs, two plenum volumes in parallel, or two in series. Lawrence Technical ran a two in parallel plenum design, tying a pair of cylinders to each plenum. Series plenums are used to equalize airflow to the cylinders on intakes where the runners are not equidistant to the throttle body. These are most common on side intake log-style manifolds found on production cars.

Intake Tuning

Tuning the intake is either modifying the system's response to different engine speeds, reducing pressure losses the air has to overcome, or adjusting the turbulence of the flow to aid in charge mixing. The most common and impactful form of tuning is changing the response to different engine speeds, or the engine's frequency response. This takes two forms: the first is in modifying the length of the manifold (ram tuning) and the second is in modifying the shape of the manifold.(including Helmholtz resonators).

The basic principle behind frequency focused tuning strategies is that a cylinder draws air differently during different parts of the four stroke cycle. Ostensibly, a cylinder only takes in air for a quarter of a four stroke cycle, meaning that there is a stop-starting of the flow of air into the cylinder. This occurs at a frequency dependent on the rpm of the engine, hence this form of tuning is done to optimize performance at a selected rpm range.

The torque curve can be viewed as a frequency response curve.

Ram Tuning

Ram tuning is implemented by modifying the length of the manifold such that the pressure wave formed on each cycle travels along the manifold and is reflected back, arriving just as the cylinder completes the cycle and takes in its next charge. When done correctly, the pressure at at the intake port will be higher during the intake stroke, giving better volumetric efficiency for the engine. The parameters to adjust are the length of your intake runners and their diameter. Intake runner length is often the easiest parameter to tune and often has the biggest impact on the system. A longer runner will resonate at a lower frequency. A larger diameter intake runner will lower the speed of the air, raising the resonant frequency for a fixed length (someone check me on this).

In more complex ram tuning applications, actuated baffles can be created to vary engine torque curve. This is done on many production cars to allow different behavior at low and high RPM.

Trade-Offs

The fundamental tradeoff with most intake tuning is to prioritize a higher peak torque or a wider, flatter torque curve.

Pros

• All intake systems have this frequency response, so you do not need extra features to enable tuning
• Relatively simple system
• Allows for movement of your torque curve

Cons

• Simulations are often inaccurate and may require on vehicle testing to confirm optimal performance
• Can only tune to one optimal frequency, unless baffles are integrated

Helmholtz Resonators

Helmholtz resonators work by having a thin neck followed by an open cavity attached to the engine's intake. Their design result in low pressure at the neck, sucking in more air, and resulting in higher pressure air in the cavity, which can then feed the engine. They can be thought of as tuned mass dampers (and can be modeled as such). Helmholtz resonators are most effective in single cylinder systems, as the flow through the intake can be dominated by pulsed flow characteristics at high rpms [citation definitely needed]. These are most often seen in exhaust systems to control the sound output.

Trade-Offs

Pros

Helmholtz tuning can be used to suppress an unwanted torque spike somewhere in the curve.

Cons

Helmholtz tuning is very hard to predict, and requires a complex additional feature that may be difficult to package and have an unpredictable or negligible effect on performance.

Throttle Control

Main page: Throttle

The throttle allows modulation of the air coming into your intake system. Within FSAE, the throttle must fall between your restrictor and manifold or, in boosted applications, between the turbocharger/supercharger and manifold. Throttle designs include actuation by throttle cable or by electronic throttle body (ETC or drive-by-wire).

Restrictor Power Limit Derivation

According to IC.2.4.1, the competition requires all air entering the engine to first pass through a 20mm diameter hole when using gasoline or a 19mm hole when using e85. This restrictor places a theoretical upper limit on the power that can be generated by a naturally aspirated engine. The upper limit is derived using the general equation for ideal compressible gas flow:

A	Area
R	Gas Constant
Tt	Total Temperature
$\gamma$	Specific Heat Ratio
M	Mach number
Pt	Total Pressure

${\dot {m}}={\frac {A*p_{t}}{\sqrt {T_{t}}}}*{\sqrt {\frac {\gamma }{R}}}*M*(1+{\frac {\gamma -1}{2}}*M^{2})^{-{\frac {\gamma +1}{2(\gamma -1)}}}$
when M becomes 1, the flow is considered choked. The equation becomes:
${\dot {m}}={\frac {A*p_{t}}{\sqrt {T_{t}}}}{\sqrt {\frac {\gamma }{R}}}(1+{\frac {\gamma -1}{2}})^{-{\frac {\gamma +1}{2(\gamma -1)}}}$
With some basic assumed values at sea level, the maximum mass air flow through the restrictor can be found:

A	3.14 e-4	m2
R	0.286	kJ/kg-K
Tt	300	K
$\gamma$	1.4
M	1
Pt	101.325	kPa

Plugging these into the equation above yields a steady state mass flow rate of 0.074 kg air per second for a 20mm restrictor. A 19mm hole allows 0.067 kg/s of air.

Some ball park power limits can be found as follows:
If we assume an AFR of 13.1 (typical lambda for high torque for NA engines[citation needed]), and a Q_LHV of 43.4 MJ/kg , the mass air flow yields a power limit of 245.1 kW or 328 hp total energy output. If the thermal efficiency of the engine is assumed to be a nominal 33%, the maximum available mechanical power is 81.7 kW or 109 horsepower with 100% volumetric efficiency.

Using an AFR of 8.7 for E85 and a Q_LHV of 29.2 (citation needed, this was just .85*qlhv ethanol + 0.15 * qlhv gasoline), the limit is 248 kW or 332 hp. The available mechanical power assuming the same efficiency as above is 75.0 kW or 98.5 hp. There are more complexities to using e85 such as the cryogenic effect of fuel vaporization leading to a denser charge, but that is out of the scope for this specific ballpark calculation

This limit can be exceeded by increasing the upstream pressure to greater than 1 atmosphere. In FSAE, any compressor must be placed after the restrictor so this is not possible.

Factors to Consider

• What is the effect of the pulsed nature of engine air flow? Is the impact of the pulsed flow consistent throughout the engine's operating range?
• what is your actual volumetric efficiency?
• are you operating at Mach # = 1 at all points in the race?

Mach Number vs Mass Flow Rate

• what is your restrictor's discharge coefficient?
  • well designed venturi style restrictors can see Kd as high as 0.95 [citation needed]
• is your engine operating at 33% efficiency? that is a rough rule of thumb

Forced Induction

Forced induction is done by utilizing a compressor to increase the pressure seen in your manifold/plenum, leading to increased engine power and efficiency. This is accomplished by the higher manifold pressures being able to force more air into the cylinder/s on each intake stroke. The increased air in the cylinder means you can burn more fuel and maintain the same air-fuel-ratio. A denser charge will also combust faster, yielding greater energy utilization and efficiency. Common methods of implementing forced induction is by turbocharging, supercharging, or procharging. Within FSAE, a fair number of teams run forced induction, but the majority remain naturally aspirated.

Turbocharging

Theory

Turbo system diagram

A turbocharger works by compressing intake air by harvesting waste enthalpy, traditionally discarded, from the exhaust. The general flow of air through a turbocharged system can be seen to the right. Theoretically, the higher plenum pressure you can run, the more power you can extract from your engine package. One limiting factor is self-detonation of the charge or knock. The liability to knock is based upon many factors, including but not limited to, fuel choice, fuel injection location, and compression ratio.

Cooling Charge Air

The charge air cooler, also known as an intercooler, is designed to lower your charge air temperature. The intake air temperature increases due to compression, according to the equation for compressor pressure ratio.

$(P_{2}/P_{1})=(T_{2}/T_{1})^{\gamma /(\gamma -1)}$

Gamma is 1.4 for dry air. This equation must be used with absolute pressure and temperature. $P_{2}/P_{1}$is also known as pressure ratio. For example, a pressure ratio of two (1 atm to 2 atm) will lead to a 20% increase in intake air temperature. This means for an inlet temperature of 20C, your plenum temperature will be approximately 85C. If you run low boost, the need for an intercooler may be able to be avoided. Alternatively, injecting E85 upstream of the runner to cool the air via the evaporation may be feasible. This is only done due to the high latent heat of evaporation and low evaporation temperature for E85. This method is beneficial as it provides cooling and well mixed air fuel mixture. However, this method requires an extremely careful tune otherwise your plenum is liable to turn into shrapnel.

Blow Off Valve

The purpose of a blow off valve (BOV) is to control your plenum/manifold pressure and prevent compressor surge by dumping excess flow. This works in conjunction with a wastegate to manage plenum pressure to your desired pressure.

Supercharging/Procharging

ETS pls

Forced Induction Manifold Designs

For Turbos:

Connecting the intake and exhaust manifolds

• Over the top
• Around the side
• I dont think anyones gone under the engine but im not willing to put money on it

avoid heating intake with exhaust headers

intercooler placement, mounting, (maybe do this in the cooling section?)