Incorporating Tesla technology into the DeLorean comes with design and engineering challenges.
The Tesla Model S was designed ground up as an electric vehicle and thus could accommodate design and engineering choices that are not available to a converted car. For example, the Tesla battery pack is attached to the bottom of the passenger cabin - in a flat slab like configuration - which also gives the Model S its excellent handling capability. Using the battery as-configured in the Model S, but in an ICE conversion is essentially impossible with significant deconstruction and rebuilding. The sections below will describe and provide rationale for the design choices selected for the TesLorean. In many cases, the design and engineering selections are compromises seeking to balance space, weight, usability, maintainability, reliability, complexity, cool-factor, and cost (in no particular order).
Table of Contents
- Drive Unit
- Drive Unit Control
- Brake Assist
- Power Steering Assist
- Air Conditioning System
- Heating and Cooling
- DC-DC Converter
- Steering Controls
- System Controllers
- Battery and High Voltage Systems
- Traction Control
After looking at dedicated EV conversion motors (e.g. NetGain Motor's Warp9) and all manner of hybrid and battery electric vehicles from OEMs, and considering the design implications of each in depth, I landed on Tesla as the target motor. There was just one problem - size. The rear motor on the early Tesla Model S cars was approximately 32 inches wide. While the large Tesla motor had very desirable performance characteristics, fitting the motor into the engine bay of the DMC-12 would have meant a complete removal and custom rebuild of the rear frame and rear suspension. This would have been a massive undertaking involving not only safety critical fabrication, but also would be likely to alter the driving characteristics and look of the DMC-12. With the stated goal of keeping the DMC-12 externally unchanged (at least to the casual observer) I needed to find an alternative solution. I briefly considered re-configuring the Tesla drive unit to position the motor length ways in the engine bay, but each change introduced innumerable new complications which detracted significantly from the beauty of using the Tesla motor.
Then came along the Tesla 'D' - for dual motor. The front drive unit on the first dual motor cars was significantly smaller than the rear drive unit. It was also less powerful, but still a substantial upgrade to the DMC-12's 105HP at the wheels. When the 70D and 90D were released in mid-2015, the front and rear drive units became identical (the large rear motor preserved for the 'P' performance versions). The new dual motor configuration has two 259HP (*battery limited) drive units on each axle. The 70D drive unit is approximately 12 inches narrower than the prior large rear drive units. The rear motor also positions the single gear transmission just in front of the main drive unit assembly (motor and inverter). The reduced size and transmission output shaft configuration meant that the drive unit could fit in the existing DMC-12 engine cradle and the driveshafts would come from essentially the same position as they do from the stock DeLorean transmission. Using the 70D drive unit meant avoiding a significant redesign of the rear frame and suspension and being able to also remove the stock DMC-12 transmission - saving further weight.
Custom driveshafts (fabricated by The Driveshaft Shop) connect the output from the Tesla 70D transmission to the DeLorean rear hubs and wheels. To the casual observer the rear arrangement of the car should remain unchanged from stock.
The Tesla drive unit requires external instruction (in the model S via CAN) to interpret the signals from the accelerator and brake pedals. The controller inside the drive unit puts the drive unit into the appropriate mode and instructs the motor. The controller responds to creep settings (should the car drift forward like an automatic ICE car when the brake is released), regenerative braking settings (how aggressively should regenerative braking be applied).
The TesLorean uses the drive unit controller from HSR Motors. The HSR module provides the data via CAN to make the driveunit believe that it is essentially still in the original model S. The module spoofs the CAN traffic that the stock controller listens for. This maintains the drive unit within the operation parameters designed by Tesla and thus safe operations.
An alternative drive unit control is available from Damien Maguire (EVBMW.com). Damien's unit completely replaces the stock Tesla controller inside the drive unit. The new controller takes in the same accelerator and brake inputs, but has a simpler and open-source set of control settings. The module purposefully does not restrict motor operations, so the motor can run in a way that generates the most power possible - potentially beyond the operational parameters determined by Tesla.
For a period prior to the HSR solution and Damien Maguire's work, Michal Elias had worked on the UMC 2.0 project. Michal's project was the first to have direct (non-CAN) control of the large Tesla motor. Jack Ricard had also gained control of an early Tesla large drive unit with CAN instructions.
Many cars (actually almost all cars, including early Tesla Model S cars) have a device called the brake booster which enhances the force of the driver pressing on the brake pedal. The brake booster uses vacuum (energy) generated by the ICE (internal combustion engine) to assist the driver pressing the pedal. An ICE is constantly 'sucking' air in (to combine with fuel in combustion) and this sucking or vacuum can be used in other parts of the car. For example the DeLorean's climate controls use vacuum to operate the various doors and flaps that direct hot and cold air onto the passengers.
Electric cars have a problem as electric motors do not generate vacuum. So to supply vacuum to the brake booster, they have in the past run a small vacuum pump. A simple pump creates vacuum by oscillating a diagram to push, pull air through one-way valves. These pumps can be noisy and they consume energy to initially create and maintain the vacuum. The vacuum created is provided to the standard brake booster device. If the driver does a lot of braking in a short time, the brake system may consume more vacuum than the pump can provide, so the system in electric cars will include a vacuum reservoir - essentially just a container kept at low pressure (lower than atmospheric pressure). The vacuum pump will run as much as necessary to keep the reservoir at the desired pressure.
The TesLorean will be fitted with the iBooster from the Model S 70D with autopilot. Bosch designed the iBooster to solve problems related to electric vehicles and vacuum supply. The iBooster does not use vacuum, rather it electrically detects the driver pressing the brake pedal and then uses an electric motor to assist. The iBooster only consumes energy when the driver presses the brake pedal. At all other times the iBooster only consumes a small standby current. The iBooster is also nearly silent drawing on an electric motor to assist. Fitting the iBooster in the TesLorean has some challenges, but the iBooster is smaller than the stock brake booster and master cylinder. The booster will need a spacer to ensure the brake lever interfaces with the DeLorean brake pedal. The DeLorean brake booster is offset by nearly 5 inches from the firewall due to the positioning of the hydraulic clutch. The clutch is no longer required in the TesLorean allowing the iBooster to be easily positioned - once the bolt pattern is accommodated.
The stock DeLorean does not come with power steering. DeLorean fans will tell you that since only 40% of the car's weight is on the front wheels that power steering is not necessary. A more likely explanation is that power steering in a rear engine car would have necessitated a power steering pump at the rear of the car (to use the engine rotation) and high pressure hydraulic lines running to the power steering rack at the front of the car. It would also have consumed additional power in an already under-powered car. Practically speaking driving the DeLorean at low speed feels like arm day at the gym. You have to haul (sometimes with both hands on the same side of the steering wheel) to pull the wheel around and effect a turn.
Even the DeLorean needs a power steering solution! There is an electric power steering solution available from DMC-EU (Europe). It replaces the stock steering column, with a new column that includes an electric motor in the driver's foot-well. It draws 12v electric power to assist the driver's steering motions only as needed. The kit currently exceeds $3000 in cost, before fitting.
The TesLorean will have a custom solution created from a readily available GM or Toyota power steering assist motor. At the firewall the DeLorean steering column connects to a linkage (including 2 u-joints) that runs directly to the steering box. The custom solution replaces the link and u-joints with a motor, an anti-vibration u-joint and other u-joints. The multiple u-joints allows the steering column to still adjust up and down, and for the motor to be flexibly bracketed out of the way of the brake lines. This solution keeps the stock steering column and stock steering box and only replaces u-joint linkage between the two. The unit requires a speed sensor (to reduce assistance as speeds increase) which is captured by a sensor installed in place of the lambda counter in the drivers foot-well in-line with the speedometer cable.
The Tesla Model S does have an electric assist power steering rack, however the unit is too large to fit into the frame channel that contains the DeLorean steering rack.
The DeLorean stock AC system used R-12 refrigerant which is now difficult and expensive to get supplies of. R-12 was phased out in the US and in Europe to protect the ozone layer. It was replaced with R-134a, still currently used in the US but being phased out in Europe in favor of R-1234yf due to climate change concerns.
The stock DeLorean system had a compressor in the engine bay, with long AC hoses (high and low pressure) running under the body (alongside the frame) to forward of the passenger cabin. The passenger side wheel well contained the AC hub, the accumulator, and links to the condenser and evaporator.
The design for the TesLorean maintains the components in the AC system apart from the compressor and the hoses. The compressor will be relocated to the front of the DeLorean (from the engine bay) to shorten the high and low pressure hoses and bring the system closer to the other AC components. This will also remove the need for AC hoses running under the the length of the car.
The compressor from the Tesla Model S will replace the DeLorean compressor which was belt driven from the engine. The Tesla compressor is an efficient scroll design and is operated by an electric variable speed motor.
The stock DeLorean passenger cabin is heated by coolant heated by the engine. Once the engine reaches a given temperature, and the driver selects a heat setting, a vacuum operated value opens and directs hot coolant to the front of the car and into the heat exchanger located in the airbox inside the passenger cabin. A small electric fan blows air over the heat exchanger, warming the air circulating in the cabin. Coolant is then routed back to the rear of the car to be reheated by the engine.
The current TesLorean design will use the Tesla PTC Heater. The PTC Heater creates cabin heating electrically using high-voltage (i.e. 400v). From desktop inspection it appears to have 6 heating levels. The Delorean heater core will be removed from the car and the airbox rebuilt to accomodate the Tesla PTC heater (which is larger). The instant heat from the PTC heater will provide for window defrosting - an important safety concern. However, if fitting the Tesla PTC heater proves impossible for the stock DeLorean airbox, then (as shown in the coolant routing below) the hot coolant from the drive unit will run to the stock heater core.
The TesLorean design will use the coolant system only to maintain the battery and drive unit within a suitable temperature operating range. The system includes a coolant heater and a coolant chiller, as well as a radiator. The TesLorean will also use the coolant chiller from the Tesla. The coolant chiller uses compressed R-134a from the air compressor to chill (or reduce the temperature of) the circulating coolant by means of a heat exchanger . This can help prevent the temperature of the coolant from getting too hot, especially if the car is not moving sufficient air over the radiator or the ambient air is not cool enough, for example on a very hot day in the middle of summer, or if the car is charging and the charger and battery require cooling.
This map shows the location of the cooling/heating system components and how the coolant routes between them. There is an outer and an inner loop (like the stock Model S design). The outer loop manages the temperature of the Charger and the Drive Unit with the fans and radiator. Since the Charger and DriveUnit are never running concurrently the overall heatload is limited to one or the other components. The inner loop manages the temperature of the Battery and the DCDC Converter. The 4-way valve connects or disconnects the outer and the inner loops depending on the thermal balancing demands of all the components. The TesLorean battery (a Chevy Spark EV A123 unit) has a built-in heater, with a heater controller integrated into the battery module controls. The battery is likely to be the most temperature sensitive of the components, in particular when the car is stationary, i.e. charging, thus the inner loop inclusion of a coolant chiller and heater. While the drive unit may create a lot of heat, this will be during driving and thus will have airflow at the radiator to remove the heat. When charging, the charger and DCDC will produce heat (likely very moderate at the DCDC since there are minor car operational loads), this heat can be removed with the radiator and fans.
The DeLorean electrical systems are only 12v systems. The battery provides 12v for starting the engine and acts as a 12v buffer as energy demands vary during driving. The stock alternator is located in the engine bay, linked to the engine rotation by a belt.
The TesLorean design will maintain many of the existing 12v DeLorean systems, however the alternator will be replaced by the Tesla DC-DC converter. The converter takes in DC high voltage from the main battery pack (in this case 400v) and outputs 12v for use by the low voltage system. The design will maintain a 12v lead acid battery to continue to act as a buffer for 12v energy demands during driving.The Tesla DC-DC converter will be included in the coolant loops as the unit produces heat as a byproduct.
The steering wheel and steering column sit inside the passenger cabin and comprise the steering controls. Once through the firewall there is the steering linkage, the steering box, and steering rack which connects to the front wheels. The DeLorean steering wheel can be adjusted up/down, and in/out. The up/down movement requires loosening a knobbed wheel at the side of the column, adjusting the height and re-tightening the wheel. The in/out adjustment is achieved by lifting a lever at the side of the column. There are no controls (buttons) on the steering column itself. The horn is activated by pressing on the end of the left steering column stalk.
The Tesla steering column is adjusted up/down and in/out though a small joystick like control at the side of the column. The column includes 2 electric motors to make the adjustments. The steering wheel includes a horn (by pressing the center pad) and two buttons and a scroll wheel to the left and right hand sides of the wheel. There are left hand control stalks for indicators, light, a stalk for the wipers and wash. On the right hand side the stalk controls the reverse, drive, neutral drive mode of the car. The steering wheel buttons control left and right computer apps displayed in the binnacle display.
Initial builds of the TesLorean will include the stock DeLorean steering column and an upgraded MOMO wheel. The Tesla wheel may be used in the future, however design of a clock spring and splined interface are necessary to allow physical steering security and data signals to be transmitted to the car from the wheel. The Tesla wheel is also 1in plus greater in diameter than the stock DeLorean wheel. This may be an issue as the DeLorean driver's position is already not generous around the steering wheel.
The TesLorean will have a number of distributed controllers that have distinct roles and which communicate via CAN (a 12v robust data network - commonly found in cars and used by ODBII). The controllers are...
- HSR Motors Drive Unit controller (proprietary)
- Battery controller (custom)
- 10kW Charger controller (open-source and customized)
- Thermal controller (custom)
- Trip computer (custom)
HSR Drive Unit Controller
The HSR drive unit controller was created by Jason Hughes (an early Tesla technology hacker). It convinces the stock Tesla drive unit that it is (still) in a Model S. The drive unit continues to take inputs from the accelerator pedal (factoid: made by Ford Motor Co in the 2015 Model S) and the brake pedal (on/off). The module operates the drive unit within the parameters originally engineered by Tesla. This will increase the reliability and longevity of the drive unit.
The battery controller is a custom controller designed to manage the Chevy Spark 2014 EV battery pack (containing A123 modules). The battery pack has two low voltage and two high voltage ports. One low voltage port connects to the BMS and receives li-ion cell data and temperatures. The other low voltage port has control lines for the high voltage contactors. The battery controller receives CAN instructions from the other TesLorean modules and sets/clears the contactors as necessary. The battery controller also publishes summary cell status information and module temps.
The 10kW Tesla charger controller was designed by Damien Maguire and programmed by Tom DeBree as a drop in replacement for the original Tesla controller. The controller monitors the charge port status and charge-ready signals. It will activate the charger when appropriate. The controller can also be instructed to charge by serial or CAN messages. In the TesLorean the controller also monitors the battery status messages from the battery controller and drive unit messages from the HSR controller.
The thermal controller monitors the coolant temperature that runs through the drive unit, battery, DCDC converter, and charger. If any unit is operating out of temperature limits the thermal controller will activate to maintain operating temperatures. The thermal controller can heat the coolant, or cool the coolant using the radiator or AC chiller. The thermal controller switches the pumps on and off as necessary and operates valves to engage the radiator or connect the two cooling loops as necessary to make optimal use of the thermal differentials available.
The trip computer interface will be a 7-in touch screen tablet located in the center console. The trip computer will take inputs from the drive-mode switches, and the touch screen. The controller will send settings to the HSR drive unit controller and the charger controller using CAN. The trip computer will also interface with the climate control system, taking inputs from the stock DeLorean controls (previously vacuum driven), then controlling relays and 12v signals to activate levers within the airbox, and also send CAN instructions to the thermal controller.
The touchscreen is the uLCD-70DT from 4D Systems. The screen module contains a graphics processor and central processing unit. It can be programmed to respond to screen touches and other inputs quickly. The screen will be paired with a micro-controller which will interface with the drive-mode switches, and send instructions to the drive unit controller and the instrument cluster controller.
Instrument Cluster Controller
The instrument controller will monitor data flows from the other TesLorean controllers via CAN. The instrument controller sets the gauge positions, controls the warning lights, and the two small OLED information displays. The binacle will continue to have stock DeLorean lights for indicators, warning lights (seat belt, doors agar, etc.).
The battery for the TesLorean will be the A123 battery pack from the 2014 Chevy Spark EV. After much research and deliberation I decided to not use Tesla modules. The Tesla modules are awkwardly sized, are relatively low voltage per module (~24v), are heavy (for 400v), and are expensive. The chemistry, while great for energy density, is more volatile than I'd like for a EV conversion. The #1 challenge was fitting the modules in the DeLorean. Unmodified would have required 16 modules to reach 400v and would have weighed in excess of 1000 lbs.
The A123 Chevy Spark EV pack contains 4 modules of 92.4v each. So the pack (at nominal voltage) is 370v, weighs 450lbs, has robust chemistry, and provides 24kWh of capacity. The pack is also configured such that with modest modifications to the pack cover can be made to sit in the DeLorean engine bay area. The TesLorean will weight approximately 3,200lbs and is expected to have a driving range of 80 miles.
The battery module from the Chevy Spark EV 2014 will be used in the TesLorean in its entirety. The pack includes a BMS, battery heater, and four 92.4V modules. Each module sits on a water cooling plate. The battery case from the Spark is made of carbon fiber and is lightweight and strong. The case will be modified to fit in the DeLorean behind the rear axle.
Here is a mindmap of the high voltage connections between the EV components.
The Tesla Drive Unit has an open differential. This means that should one of the wheels slip (not be able to grip the ground) for any reason (water, ice, gravel, etc.) the other wheel may not spin at all. The ability for different wheel speeds are actually a design feature. When a car is going round a corner, the inner wheel is traveling a shorter distance than the outer wheel in the same amount of time. This means that the outer wheel is traveling at a faster speed than the inner wheel. An open differential accommodates the need for the wheels to be able to drive at different speeds.
The downside however is when a wheel experiences a loss of traction. Should one of the wheels be on a patch of ice, water, or gravel, it might slip on the surface. When this happens with an open differential - all the power from the motor/engine goes to the wheel that is slipping, and little or no power goes to the wheel that is not slipping. This will result in the car not moving, but with one wheel spinning at high speed and the other (the one with grip) not spinning at all. This can be a dangerous condition if the sleeping wheel suddenly recovers grip - causing the car to potentially move quickly and unpredictably.
Traction Control is the mechanism by which the drive line engineering ensures that if a wheel does slip, the slip is limited in some way, and results in transfer of the power to the wheell that still has traction. While the Tesla has an open differential (subject to traction loss issues), the problem is contained using two mechanisms 1) reducing power when traction loss is detected, and 2) momentarily applying the brake to just the slipping wheel. Reduced power reduces the likelihood for slip, and applying the brake to the slipping wheel will cause more power to be directed to the non-spinning wheel that has maintained traction. Traction control systems are normally highly integrated into the fast response anti-lock brake, and stability control systems. Since thse don't exist in the DeLorean another solution is necessary. A back-heavy DeLorean with a open differential could be prone to loss of control when traversing ice, wet roads, gravel, or even uneven pavement.
The solution is being sponsored by Zero-EV.co.uk. They have commissioned Quaife to develop a ATB (automatic torque biasing) limited slip differential for the Tesla small drive unit. The Quaife ATB replaces the stock Tesla open differential inside the drive unit. The ATB limited slip differential allows the wheels to spin at different speeds for a range of speeds (which is required for cornering etc.) but then progressively limits slip at higher speeds. This is a mechanical solution and does not require advanced monitoring or control systems.
Credit: DeLorean DMC-12 Blueprint Engineering and Technology Magazine