Electrification of a Lotus XI replica

Background

I first built the Westfield XI about 4 years ago. I had dreamt of having a Jaguar D type but settled on something I could afford but still gave me the 50s Le Mans feel. The basis is a space-frame chassis with fibreglass and aluminium sheet bodywork. The running gear and engine/gearbox came from a '68 MG Midget I had scrapped. I loved the car for the first year. Then I wanted more power. So I tuned the engine.......this made it more unreliable and noisier. The rebuild also introduced an oil leak from the rear main bearing seal - a known weakness of the 1275 A series engine. Out came the engine for the 3rd time for an update to the rear seal. I then blew it up trying too hard on a track day. So I started to look at alternatives.

 

I wanted performance and reliability. My wife and children wanted the car to not smell, be quieter and not emit harmful gases. So started the electrification journey.

I had often talked about converting a car to electric but had never had the time to do it. So I set myself a plan to complete it in a year. I had experience of engineering electric vehicles through working at Ford Motor Company and through my time living abroad I had contacts in the electrical component supply world - I started to explore the possibilities. This was the start of ECOClassics.   

Specification of the electrical system

I wanted to try to maintain a similar power and weight to the original car and to optimise the weight distribution through placement of the batteries. I set a target of 130 mile range and a sub 6 sec 0-60 mph time (the petrol powered car achieved 90 miles on a tank and 0-60 of 8 seconds).

I picked a water cooled motor with matched inveter that delivered 60kW (80PS) peak power (similar to the original A Series engine) and 200Nm of torque. The overlay of the power and torque data showed similar peak power but a very different delivery of a much higher torque. The motor would replace the gearbox and directly drive the propshaft.

I plotted out the torque at the wheels vs road speed for both the direct drive electric solution and the petrol engine with gearbox. Clearly the 1st gear performance of the petrol out did the direct drive electric motor due to the torque multiplication of the gearing. I still felt it was the right solution for performance as there would be no gear change to lose time and the performance off the line with no clutch modulation would be superior plus the instantaneous torque at very low RPM would give the direct drive an advantage.

 

The data was run through a performance CAE tool to calculate the likely performance characteristics. These confirmed that our targets could be achieved - we played with different final drive ratios and clearly a move to a shorter final drive (4.55 instead of the the standard 3.91) would deliver a sub 6 second 0-60 and still be capable of well over 100mph (a speed I have not seen with the petrol engine...........).

 

The control box is a combined unit with the motor controller, AC inverter and DC/DC transformer all in 1 box with air conditioning and PTC heater control if required. We picked a 3kW charger so it would not need to be water cooled (saving the complexity in the build) and the plan is to only charge this off the household 13A system for now.

The package space for the batteries was measured once the engine and gearbox had been removed and we calculated that we could distribute a 330V, 33kW battery, split 67% front and 33% rear. This helped the balance of the car and enabled a sensible package. To aid the build and package, the batteries were split into 7 units of 50V each, to be connected in series. With this design work completed we ordered the system components.

Prototyping the installation

To speed up the build process and enable us to make any adjustments early we used the supplier CAD and 3D printed the motor and inverter unit. We also used the CAD to design the motor mounts which were also 3D printed. Trial installation of these enabled us to build a sub-frame to mount the motor and inverter ready for when they turned up. 

Prototype battery packs were mocked up from wooden blocks to enable subfames to be built to hold the batteries in place.

 

System sourcing

The complete electric powertrain system and batteries comes from China. We have worked with an integration supplier to pull the components together from suppliers we have designated. The motor and controller are used in OEM cars in China whilst the rest of the components are pulled from OEM suppliers across the automotive sector. We have met these suppliers on several occasions to ensure they can meet our specification and quality requirements. The battery pack is our design with assembly completed by the integrator. We ensure that the integrator assembles the components into a system test rig to give the system a full workout before shipping the complete system to the UK.

Installing the motor and inverter

The motor is situated right where the gearbox was and is a tight fit in the chassis. I wanted to keep it as low as possible in the car whilst maintaining as straight line in the driveline as possible, the other constraint was the ability get to the cooling connections. The 3D printed motor and mounts enabled us to ensure these criteria were met and have the motor brackets and mounting plates laser cut and fabricated prior to the system delivery – thus speeding up the build. The motor mounts are simple plates bolted to the front and rear faces of the motor. These then bolt directly on to brackets welded into the chassis. I opted to hard mount the motor for several reasons (package space, simplicity, speed of build). I was acutely aware that if there was anything out of balance in the driveline then this would be directly transferred into the body. Test on the motor both prior to installation and once installed showed that it was super smooth and did not excite any natural frequencies in the chassis or body, so we continued with this design. With hindsight I would probably isolate the motor with rubber mounts on a subframe. I would do this primarily so we can absorb some level of driveline vibration, it would also help with shock loading of the driveline when delivering full torque on a pull away (more on how we coped with this later) and it would make the process reversible – a really important factor for a classic car conversion but not a consideration for me with this replica as I am totally committed to the electrification of my car.

So, with the motor installed we moved on to the big box of electronics. The box contains most of the systems needed to run the car. It has the inverter to take the DC battery output and convert it to 3 phase AC output to run the motor. The motor controller is integrated into the package too, along with the DC/DC converter which acts as the alternator replacement delivering 12V output to charge the 12V battery and power all the vehicle side electrics like lights and instrumentation. The box also has the capability to run a PTC heater (an electrical heater matrix which would replace the water matrix in the heater box on a petrol/diesel car). It can also power and control an AC compressor. Given that the car has no roof, neither of these creature comforts are fitted.

The key constraint with placing this unit was to keep it as close as possible to the motor to reduce losses and heat generation in the connection between them. The natural place to site it was directly above the motor with a vey short run of this 3-phase wiring. This position also helped with the cooling connections and high voltage cable connections. The brackets to hold the box in place were welded on to the front chassis crossmember and the cross-car braces across the scuttle area. An interesting by product of fixing it to the scuttle was an improvement in scuttle shake on rough roads – a welcome benefit. Whilst this package worked for this car, I can see that it would not work on many due to the size of the unit. The individual components are available and using them in this car would have meant I would not have had to cut the bulkhead on the scuttle cover – so probably a better route if I had wanted to hide more of the ‘guts’ of the electrification. As it happens, I like the way the motor connection comes through the bulkhead with the wires running down to the motor.

The Battery

The next parts to be installed were the battery boxes. Before I describe the installation, we should take a look inside the box and understand the battery construction. The basis of the battery are lithium ion consumer cells. They are 18650 batteries as found in many consumer products such as torches. They are similar to those used in TESLA model X and S batteries. The batteries are arranged in a sub-module of 38 batteries in parallel. The individual battery terminals are welded to plates inside the sub-module which then provide the terminals for the sub-module to be wired up inside the full battery module. The sub modules are bolted together to form the battery module. In our case there are 12 sub-modules in a case giving a nominal voltage of 45V. The box also contains a battery management system (BMS) data collector which connects to each submodule providing voltage, current and temperature information for that sub module. The battery modules are isolated from the steel casing and have the HV connectors and BMS connector on the front face. The 7 batteries in the car are then wired in series to give a nominal 315V DC battery. There is a fuse in one of the battery packs that protects the whole battery. The 7 batteries are arranged 4 in the front and 3 in back to help package them and get to an optimum weight distribution. With hindsight I would build the battery as just 2 packs to avoid the weight of the multiple battery boxes and the complexity of the high voltage wiring whilst also enabling both batteries to be protected with a fuse/master disconnect.

Installation of the Batteries

​The key consideration for the HV battery packs is for them to be retained in the event of an accident. I have built frames from angle iron to bolt between the battery mounts and the chassis – extremely strong but probably a bit on the heavy side. As the batteries are in the space where the engine was, I feel comfortable that they will be protected in an impact. I also reviewed the ECE requirement for battery homologation (ECE R100.R2 Part 2) and whilst the battery packs have not been tested to the requirement it states that the battery would need to be 420 mm rearward of the front of the vehicle and this is met in the car.

All things charging

 

The charger is a 3kW air cooled unit that takes AC from the mains (over a standard 13 amp plug) and converts it into around 320 V DC. It is connected into the inverter unit and is controlled over the CAN by the VCM. Using a 13 amp plug give me the opportunity to charge at anyone's house but the charge time for a full recharge is around 9 hours.

 

I could install 7kW on board charger, this would need a 32 amp wall box charger to run it (this is the maximum you can achieve on a single phase home supply). Whilst this would provide faster charging the drawback is it would need water cooling for the charger on the vehicle and whilst possible I decided not to build it in yet as it would slow down the build and the difference in charge time in the context of this car and how it is used is not an issue. System connection points and mountings for the 7kW on board charger have been built in allowing an easy update at some point.

 

The charger is mounted in the side pod where the fuel tank used to be with open access to airflow but out of any direct water spray path. I spent a long time debating where to put the charge port.......in the end I opted for the easy route and put it in the trim beside the driver. As the car is permanently open topped, access is easy and the port is inconspicuous in this position and well protected. I did want to put it in place of the fuel filler cap but the small diameter of the filler mounting meant I would have to make a new larger adaptor and mount. A project for the future given the amount of machining to get the charge socket to fit and remain watertight. 

TOPICS STILL TO COME:

All things cooling

The driveline

High voltage wiring and battery monitoring system

Low voltage wiring, shifter, display, 12V battery and CAN

System test

Road test

Initial data analysis

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© 2020 by Nick.