Explained: Mitsubishi i-MiEV With JB Straubel’s Extended Range Pusher Trailer
Being born of a petroleum engineer, I spent my childhood bouncing around the globe chasing black gold, so there’s some irony in my infatuation with electric vehicles. I still enjoy playing with engines, especially when that dichotomy enables a hookup with my first love, the air-cooled Volkswagen! I read about the pusher trailers built by Mr. Sharkey and JB Straubel back in the early days of the EV Discussion List and saw the immediate practicality of such devices.
Editor’s Note: Jay’s story is in response an earlier InsideEVs’ article on the curious unit, that only brought up more questions about the pusher and its origins. Our thanks to Jay!
One could build a high-performance but limited-range EV for daily use, and hook up a proven fossil-fueled drivetrain when needed for highway journeys! Armchair engineers and knee-jerk reactionaries are usually quick to proclaim such machines to be inherently unstable time bombs (especially Volt drivers- why so defensive?), but having trailering experience in the 400 lb to 20 ton range, I thought otherwise.
Battery-based range trailers get heavy in a hurry, and have high capital cost, and generator sets large enough for continuous highway driving (at least 15 kW) are heavy and spendy too, with the added difficulty of designing an electrical interface that won’t blow both your OEM electronics and your vehicle warranty. So I resolved to build a pusher trailer and assembled a tidy plywood wedge trailer around a VW transaxle and Type III VW subframe, with the added benefit of a foldout platform tent on top! See the last two photos in the following collection from Seattle EV Assoc Ex Officio Steve Lough.
However, before I managed to get a salvage VW engine running for it, I found the holy grail via online EV forums: the original aircooled VW pusher built by Tesla’s JB Straubel himself!
It had been hibernating in a Palo Alto backyard for over a decade in the hands of its third owner, who had upgraded to a Tesla so was long past any need of a pusher. I negotiated its donation to the Historic Electric Vehicle Foundation, but was granted use of this item until a museum exhibit is ready to receive it.
By design, a pusher trailer is not used for around-town driving. It is meant for highway use. Even so, testing has proven the theory that the pusher’s light weight compared to the tow vehicle does not make it any more prone to jackknifing than a conventional trailer, and the use of a long tongue lends leverage against the pusher’s pitch and yaw. It also places that noisy engine as far as possible from the driver’s ears and makes it easy to back up, but induces a bit more drag by protruding beyond the tow vehicle’s wake.
This pusher trailer went through several Straubel iterations, finally with a small servo to control the throttle and a second fuel tank, though both of those were too badly deteriorated to put back into service. After testing another hobbyist’s aircooled VW pusher, I became convinced that throttle control was no longer necessary.
With a series-motored conversion like JB’s Porsche 944 or my Karmann Eclectric, there is no regenerative braking to make use of excess power and scrub off speed, so one must either heat up the brakes (the very essence of inefficiency) or turn the engine off on every deceleration. This provides instant full engine braking, but makes for a rather rough ride.
With a modern EV that has strong regen and user control of regen intensity like the i-MiEV, surplus power can be put into the battery instantly. At 33 kW, the i-MiEV regen packs more watts of recovery per pound than almost anything not built by TESLA, and there are four user-selectable regen intensities, plus peak regen can be tapped in what would be the brake pedal deadband on conventional cars.
The current pushing mode is in a word, binary. The engine is either pushing at a predetermined throttle setting, or in full engine braking. During testing I gradually eased up to wide open throttle, and have found that this enables continuous state-of-charge highway cruising at 65 mph, with heavy regen on the downhill sections, and electric boost required to maintain speed when going uphill.
The engine ignition current is supplied by the tow vehicle’s accessory outlet (cigarette lighter plug), and can be turned on and off by simply pulling the plug. I built a control pendant with a lighted toggle switch to show when the ignition is hot, and one can add a third layer of control by placing a reed switch next to the brake light wire, causing the engine to turn off whenever one hits the brakes. I wanted to use the highest level of regen, available in the iMiEV brake pedal, so have not used this option.
Also, when pushing in 4th gear, the engine torque is not much of a match for the vehicle brakes. Full regen slows the vehicle down faster than the usual flow of traffic, and the engine loses much of its torque below 40 mph and stalls out completely if the ignition is left energized below about 30 mph, at which point it switches to full engine braking.
Under binary control, the most awkward part of using this pusher is pulling over at the top of a freeway on-ramp and shifting the manual transmission from Neutral to Fourth gear. The pusher bucks and complains for a couple of seconds, but then spins smoothly and silently until one energizes the ignition at 30 mph or above. Then the car can accelerate from 40 to 80 mph like a rocket, having exactly twice the power of a stock i-MiEV (or a stock VW Beetle) on tap.
Driving dynamics on the highway are superb. In town, the trailer bounces and jerks on the hitch like any ordinary cargo trailer, but when power is applied, everything smooths out and the ride is superior to an unladen i-MiEV and better than any trailer I’ve ever pulled. Those of you who pull trailers with strong brake systems know the feeling: the trailer is smoothest when you are slowing it down or accelerating. Pushing provides a similar effect.
But what about CHAdeMO? In my experience-based humble opinion, dc fast charging is great for turning a local car into a regional car, but more than one or two fast-charges on a trip take too much time and introduce too much risk to make an i-MiEV or LEAF practical for many trips beyond about 100 miles.
Aerovironment’s decision to price themselves out of the Seattle market was one of the final straws in my decision to pursue a pusher trailer. Why pay $20/month for a service that you only use on a few journeys per year, due to the huge hole in their territory? Paying $7.50 for 40-45 miles of travel a-la-carte is just plain stupid. Of course, TESLA seems to have hit a Supercharging sweet spot, but the rest of us just have to wait for that sweet spot to be economically attainable.
However, pushing at full throttle can’t possibly provide the best fuel economy. This engine hasn’t been tuned for full-throttle operation, and there must have been a few pits put in the cylinder walls during its decade-plus of outdoor hibernation. Exactly in line with the other aircooled VW pusher I’ve experienced, this one gets 20 mpg on long trips, which always include some hilly terrain in Western Washington. When using the pusher just as a mid-range extender, the effective MPG rises significantly (as with a plug-in Prius kit). For instance, if I drive 45 miles on electric and then push for another 45 miles, the car returns 40 mpg while also returning a small increase in the depleted battery charge, depending on speed and terrain. I hope to improve upon this by re-tuning the engine and adding back a servo on the throttle. I’m also interested in reducing emissions via either a propane conversion or a catalytic converter. However, a good air-cooled VW engine isn’t as bad as most folks think. Unburnt hydrocarbons are relatively low for a vintage engine, though NOX tends to be high due to the higher combustion temperatures of air-cooled engines.
Several folk have built engine-powered pusher trailers out of FWD automatics like VW Rabbits and the Honda Civic . These offer the advantage of a seamless transition from city to highway mode via remote start and automatic shifting, and the same situation applies regarding throttle control, binary vs variable.
I have a genset trailer too; though it only supplements the range of my homebuilt EV due to the uncertainties involved in hacking OEM systems for on-the-fly charging. The last few years have seen many attempts to create genset trailers that will interface with OEM vehicles, such as the French EP Tender, but require extensive customization for each vehicle that will pull it. I wish them well, but their high cost and the small target market has not allowed any market success to date. An agnostic pusher like this ugly antique can quickly transfer to any vehicle with a trailer hitch, so I hope that no overzealous regulator deprives our small group of hobbyists of the ability to refine this technology. It is extremely useful and has potential for wider and greener application.