Throttle

Throttles are a mechanism in the intake system that can control the amount of air entering the engine and modulate it's output. To control the air flow, a valve is opened or closed. The specific design of the throttle system can impact the characteristics of the vehicle.

Size

Throttle sizing is a game of compromise. If the throttle is too small, the air flow will be unnecessarily restricted even when the valve is wide open. If the throttle is too big, the air flow will be restricted by the restrictor before the throttle valve is at 100% open, reducing the amount of control the driver has over the engine. An over sized throttle will also be slower to respond to driver input and the added size and weight may complicate integration.

Throttle Form Factors

Butterfly

Throttle Area vs Alpha

Butterfly valves are the most common throttle valves used in industry and automotive racing.
As butterfly valves open, the cross sectional area does not increase linearly with the throttle open percent. The opening ideally changes as a function $1-cos(\alpha )$ where $\alpha$ is throttle angle. In the real world, the throttle valve must be attached to a shaft that becomes the dominant obstruction to airflow at higher throttle angles. Where the shaft becomes the significant obstacle depends on both the throttle and shaft diameters. The relationship between your throttle angle and true air flow can be found on a flow bench or a validated simulation if the manufacturer cannot supply the data.

The truncated cosine function gives the butterfly valve an inherent progressive throttle response: the rate of throttle opening increases as the pedal is pushed. This translates to fine control at low throttle angles and a quick response to high throttle angles. However, some drivers may desire a more progressive or less progressive throttle than inherent to your system. This is a tunable parameter by swapping out the cam on the throttle body itself or adding a cam to the pedal. A progressive throttle response combined with certain engine tunes or tuning strategies can produce a jump in torque felt throughout pedal travel known as throttle snatch. This can be tuned out or "tamed" with a cam on the throttle body or pedal. Commercial cams and cam-tamers are available for motorcycles and can easily be mimicked or adapted for use in FSAE.

An electronic throttle control system (ETC) can be used to create and easily modify a function of throttle pedal % to $\alpha$ of any desired shape.

Barrel

Barrel vs Butterfly throttle response curves for a fixed diameter

Barrel Throttle are not as common in FSAE but have been used. Teams who run them usually cite the lack of restriction at the higher end of throttle angles present in butterfly valves. This means a smaller diameter barrel throttle body can be used than a comparable butterfly valve, though it should be noted that the barrel-style throttle will likely still weigh more. They also have a more linear inherent throttle response compared to the progressive nature of the butterfly.

These throttles are harder to manufacture as they require tighter tolerances over larger areas. They are also significantly heavier for a fixed diameter and the valve itself has a higher rotational inertia.

Slide

Like barrel throttles, slide throttles are less common but have appeared in competition. Unlike barrel or butterfly valves, slide throttles do not have an inherent throttle response curve. How the throttle behaves depends on both the shape of the aperture and the opening action. They can be opened linearly or rotationally. Slide throttles are used for their simplicity, low weight, ease of manufacturing, and low profile.

Iris

Iris throttles are uncommon in automotive and FSAE alike. Iris throttles have an exponentially progressive response as the area is proportional to $r^{2}$

Throttle Return

To ensure safety in the competition, the throttle must return to the closed position if a failure occurs. Teams are required to have at least 2 redundant throttle return springs. These may be of any design, but must be provably independent. The springs must not be interlocking, or interfering. Some throttle bodies come with an internal, hard to access or hidden spring, which may be used as one of the return springs as long as it can be shown to work by itself.

Idle Control

The throttle cannot be completely closed or the engine will shut off. To maintain an idling engine, a small amount of air must be allowed. There are a few ways to do this, including but limited to:

Cable Stop

A crude method of maintaining an open throttle is to restrict the closing of the throttle by shortening the throttle cable to disallow the complete return of the throttle. This frequently leads to an inconsistent idle and can vary based on how quickly the pedal is released. However, it is cheap and simple and the inconsistent TPS can be compensated for in the engine tune.

Valve Hard Stop

Some throttle bodies come with an adjustable feature that gets in the way of the valve closing completely. Often this is a set screw. This can be tuned while the engine is running and yields extremely repeatable results.

Idle Air Control Valve

A third method is to have a secondary opening in the throttle body that allows air into the engine. This smaller opening has a valve that allows tunability, but is not changed during the race. This allows the primary throttle valve to be closed completely. This method is the most consistent as it is not subject to driver input. However, the throttle body becomes significantly more complicated.

An idle air control valve can also be electronically controlled by ECU and can even compensate for ambient conditions and engine temperature.

Actuation

There are two main ways to translate driver input into throttle output. The most common is by physically connecting the accelerator pedal and throttle by a cable. The second way is to use the a sensor to read the pedal input and a small motor on the throttle to open it by the desired amount.

Cable

If using a cable actuated throttle, the choice and routing of cable are important considerations. The most common cable used is a thin steel cable as found on a motorcycle or brake or shifter cables from a bicycle. A push-pull cable an also be used and has negligible stretch/flex over time, but weighs much more.

Other popular choices are shifting and brake cables used on bicycles. Brake cables are often thicker than shifter cables. A steel cable will always have some stretch, and a thicker gage will stretch less. The housings supplied for brake and shifter cables are also different. Bicycle brake cable housing has steel reinforcement wrapped in a spiral along the length of the housing. Shifter cable housing has longitudinal reinforcement that leads to the cable being incompressible, but the minimum bend radius can be compromised..

Cable fittings are either swage or swageless. Swage fittings are permanently attached by crimping (swaging). Swageless fittings can be attached to the cable in a number of ways including set screws or collets. Swageless fittings allow for a degree of adjustment as the cable stretches.

Lubrication

Cable lubrication is a topic that will provide varying answers from all sources, largely anecdotal and often conflicting. To complicate matters, the specific type of cable used, the stroke, the bend radius, the environment, the sheath material and the cable's cost will play into the specific lubricant strategy for any application. The lubrication of throttle cables in the FSAE application is often more effort than it is worth and does not result in a point difference in dynamic events. When done incorrectly, the throttle cable can gum-up and become sluggish, stuck, or damaged. As inexpensive bicycle type cables are very common in FSAE, it is often cheap enough to simply replace the entire cable as needed instead of inspecting, tuning, and servicing the same one at regular intervals.

However, for the sake of longevity, safety, and Murphy, attention should be paid to the specific application on your vehicle. Some cables do not require any lubrication, please refer to the manufacturer recommendations for correct lubrication and service. If the information is not supplied by the manufacturer, dry lube like graphite or molybdenum disulfide are the best go-to for high quality cable sheathing. Wet lubricants can attract and hold dust and particulate matter that can damage the sheath lining.

However, if the cable is in a wet/humid environment or the sheathing is old or slightly damaged, a liberal dose of a high quality oil such as trans or motor oil is desirable[citation needed]. Non-synthetic oils like mineral oil can degrade the cable housing [citation needed]. WD-40 may also degrade the housing and will dissolve the lubricant applied to the cable by the manufacturer and possibly the cable lining[citation needed].

Further reading here: https://www.sheldonbrown.com/cables.html

Electronic Throttle Control

Main page: Electronic Throttle Control

overview and driving rationale how it fits into system not specific design info