Small-Engine Vehicle Design Project

Overview

The go-kart was built by combining the engine from a broken pressure washer with a riding lawn mower missing an engine. Many modifications had to be made to the frame of the lawn mower to accomodate the new engine.

Modifications & Design Decisions

Centrifugal Clutch
To allow the vehicle to overcome inertia when stopped, a centrifugal clutch was employed. A centrifugal clutch contains a material with a high coefficient of friction, which is spring-loaded and separated from the inner walls of the clutch at rest. When the device reaches a certain threshold RPM, the material makes contact with the inner wall which connects the inner and outer rotors. This allows the engine to spin independently of the wheels at idle. When at operating speed, the clutch fixes the wheels to the engine.

The centrifugal clutch was chosen for the sake of simplicity; having this type of clutch eliminated the need for a complicated drivetrain involving a separate gearbox and transmission. It provided a simple solution to the engine stalling when stationary. The main drawback of the centrifugal clutch is that it prevents the entirety of the engine's power from being taken advantage of. The simplicity of a fixed gear ratio comes at the cost of low torque during acceleration and high engine RPMs during cruising. Even with a fixed gear ratio, during extended runs the vehicle was able to reach speeds in excess of 60 KM/H.

Chain Transmission
In order to transfer power from the engine (clutch) to the wheels, a chain and gear system was utilized. In this configuration, a motorcycle chain was stretched between a gear on the clutch and a sprocket that was connected to the rear axle of the engine. To allow the chain to pass through the frame, a rectangular hole was cut.

Throttle Control and the Governor
As a result of repurposing an engine from a pressure washer, manual throttle control was not possible without modification of the engine. Pressure washer engines are designed to operate at a constant RPM with varying torque. To achieve this, a governor is used. The governor uses mechanical centrifugal feedback to operate the throttle through a spring. Rather than remove the governor entirely, the spring was simply removed and replaced with a cable connecting the engine's throttle directly to a pedal on the frame.