Installing the Fuel System and Engine

A 4-gallon aluminum fuel cell was purchased and adapted to fit in the car. No mount points were included on the fuel cell, so four brackets were waterjet from 5052 aluminum and welded onto the tank.

Corresponding mounts were welded onto the vehicle frame. The tank also didn’t include a fuel level sensor, so a universal unit was installed. The stem of the sensor’s float was trimmed to work with the height of the fuel tank and a hole was cut into the top of the tank with a hole saw. Five holes were drilled and tapped, and the level sensor was screwed into the tank. Finally, the tank was installed into the vehicle with four bolts.

Two fuel pumps were used in the car. One is attached on the engine itself, pressurizing the fuel rail. The second, the lift pump, was mounted just after the fuel tank. A simple steel bracket was cut out and welded to the frame. The pump was bolted on and a wiring harness was made to connect it to the power supply from the engine’s circuitry. Quarter-inch rubber hose and a number of fittings were used to connect the tank to the lift pump and the pump to the engine.

The engine mount, being interconnected with the intermediate shaft mounts, was already tacked into the rear of the car. After test fitting, the mount was welded fully into the car. The engine was seated on the mount and four bolts were installed. The primary clutch of the CVT was slid onto the engine’s shaft and the end bolt was torqued on. The V belt for the CVT was installed over the two clutches.

The throttle for the engine was mechanical, so a system was made to take electronic inputs from the pedal and actuate the throttle. Of course, eventually, this system will be controlled by the hybrid control algorithms, rather than via a direct connection to the pedal sensor. A high-quality, titanium-gear servo was chosen as the actuator. An aluminum mount was cut and welded to hold the servo near the throttle. A tiny set of ball joints and a pushrod connects the servo arm and throttle.

Finally, a simple circuit board was made by lasercutting a pattern and etching away copper with ferric chloride solution. This board together with an Arduino read the pedal signal and transmit a PWM command to the throttle servo. At this point the car is driveable in gas-only mode.

 

Fabricating the Drivetrain

The power from the gas engine and electric motor is delivered to the wheels through an intermediate shaft and a Miata differential. The mounts for the longitudinal intermediate shaft were already tacked to the car during frame fabrication.

Aluminum bearing mounts were turned on a semi-CNC lathe. These parts allow a press fit for the bearings and adapt to the frame mounts via a three-bolt flange. Into a 6-inch diameter aluminum billet, a pilot hole was drilled and then a boring operation was performed to bring the diameter to an interference fit with the bearing.

Once the hole was completed, a step-down was cut to assist in locating the adapter to the mount hole on the frame. Then, using a center-finder, the part was positioned on a semi-CNC mill. A three-hole bolt circle was drilled through the part. This process was repeated for both intermediate shaft bearing adapters and for one differential bearing adapter.

Finally, the bearings were pressed into the adapters with an arbor press. A small amount of epoxy was used to ensure the press fit wouldn’t release with vibration.

The intermediate shaft was made from a 3-foot long 1-1/2-inch chrome-plated steel shaft. The shaft passes through two of the bearing assemblies and interfaces with three different components. It receives torque from the engine through a belt-drive CVT. The secondary clutch of the CVT has a 1-inch bore with a 1/4-inch keyway. The last 120 mm of the intermediate shaft was turned down on the lathe to allow a slide fit for the clutch. Also, it was cut to length, faced, and chamfered.

The shaft was then fixed in the mill and the 1/4-inch keyway was cut using a programmed path. The shaft also receives torque from the electric motor and transmits it to the differential through two chain drives. The purchased sprockets already had 1-1/2-inch bores and 3/8-inch keyways, so two more keyways were cut in the opposite end of the intermediate shaft.

To allow for geometric imperfections in the frame’s mounts and to absorb vibrations from the rotating components, bushings were lasercut from a sheet of Buna rubber. The three bearing assemblies were bolted to the frame, sandwiching the bushings, and the shaft installed.

The differential was mounted by its carrier bushings in the rear and by two 3/4-inch bolts in the front. The mounts were already tacked onto the frame. So once the positioning was confirmed, these were welded fully. The differential was lifted onto a dolly and bolted into the vehicle.

The differential receives all the torque from the intermediate shaft through a flat sprocket bolted to its four-bolt flange. The four holes were drilled on the mill and the center bore was turned to 3/4-inch. A steel hub was also turned to reinforce the connection. This was bolted to the differential and a 3/4-inch stub shaft was inserted through the bearing and through the hub.

All bolts were then treated with red thread locker and torqued. The half shafts were installed into the differential and into the rear hubs and the wheel nuts torqued. Finally, a quart of synthetic gear oil was added to the differential.

Installing the Brake System

The brake system consists of a single pedal, two master cylinders, a bias valve, steel hardlines, PTFE flexlines, and four 4-piston calipers.

First the calipers were installed onto the uprights using adapters from Flyin Miata. The rotors were installed on the hubs.

Before the hydraulic system was fabricated, the mechanical handbrake was installed. An off-the-shelf parking brake handle was used and a steel mounting bracket was waterjet and welded to the frame. A steel bulkhead plate was made from a quarter-inch and 12-gauge steel. The ends of the parking brake cables slide through two holes in the plate and are held in by a couple c-clips.

After bolting the parking brake handle in and welding in the cable bulkhead, a simple bracket was designed to attach the cable ends to the handle. Finally, the cables were installed and attached to the rear calipers.

Then, the aluminum swing-mount pedal from Wilwood was installed. A tube was cut and bent and a number of steel eighth-inch brackets were cut and tacked in. After checking the pedal position, the mount was welded completely.

From there, the rest of the brake system fabrication was plumbing. A bias valve was installed on the front brake lines, since the master cylinders were chosen to favor the rear-heavy vehicle. Also, a tee was installed in the rear line to connect to a brake light switch and a brake pressure transducer, to be used later for motor regen. The front hardlines were bent, flared, and tightened. Thin steel brackets were welded onto the frame to mount the flexlines with c-clips.

The rear brake lines were constructed similarly, although with a little more difficulty. Since these were longer, it was tricky to get the bends correct enough to keep from pulling on the flare connections when tightened. With those installed, the brake system was complete.