"Pusher" Trailer Design / Control Logic

This thread is to discuss an internal combustion reciprocating engine that is trailer mounted, designed to push an EV. This thread is NOT to discuss the merits of such a venture (which I personally find nebulous). It is a design exercise.

For the discussion, we will assume this apparatus is street legal and CARB - EPA compliant. It can have 1, 2, or more wheels.

PRIMARY PURPOSE: the device could completely charge an EV with regen while being towed, as well as propel the car as far as the fossil fuels keep getting replenished.

A) A traditional VW "Bug" transaxle and motor (yes, it could be ANY drive engine, but this focuses attention to design and not motor selection). Further, assume this motor has adequate power to propel the tow vehicle up hills (let's say 40hp) and it is NOT a diesel (LPG, CNG, or gasoline).

B) The ONLY physical connection to the vehicle is the 2" receiver hitch and the 4 wire trailer lights.

C) There is absolutely no modification to the tow vehicle beyond a traditional trailer hitch and proper wiring for trailer lighting (license plate light, brake, running lights, left and right turn signal)

D) A cable with a Cockpit Control Box (CCB) must be provided for access while he/she is operating the tow vehicle. It will contain a power button, start button, kill switch, basic motor indicators, and a throttle control.

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1) Flip a switch on the CCB to power up the pusher electrics. Electrical power comes from the 12 volt battery on the trailer, which is also used to power the Bug motor's starter.

2) Press the start button, which is not enabled until the pusher electrics is on.

3) The motor won't start unless the clutch is disengaged. Anytime the clutch disengages, the transaxle goes to 4th gear. This requires two servos or linear actuators. The power switch disengages the clutch and puts the transaxle in 4th gear, and enables the start button (N/O relay closes to allow power to the starter solenoid switch).

4) With the motor running, the simple way to engage the clutch is to drive the tow vehicle to the calculated speeed that the fast idle speed of the VW motor will be in 4th gear, then engage clutch. Ideally, this would be an automated process that when the motor flywheel speed and clutch speed would match within X%, which would then engage the clutch.

5) This would enable the pusher throttle control. The vehicle can remain in Drive, or "ECO" / B / L, which will recharge the battery in the EV, or neutral to let the pusher just propel the car.

6) Pusher throttle control - I think a boat or airplane type lever, but the actual connection to the motor throttle needs to be a "fly-by-wire". No clunky cables.

7) Pressing the tow vehicle brake pedal will open a NC switch to stop the Bug motor from propelling the vehicle (separate from the kill switch)

8) To disengage / shutdown - Hit kill button which will disengage clutch and put transaxle in neutral.

9) Turn off the power switch, which will shut down the motor by cutting 12 volt to the ignition coil, and shutting off the Fuel Shut Off Valve (FSOV).

10) Optional trailer brakes.

What is the practical use of this? And how much would it cost instead of a tow? I am assuming this is the push the car when dead? Or is it to provide additional power for hills reducing the need to drain you battery?
Is this even street legal? Is a tow hitch the best connection for this?

I know, I know you said don't discuss the merits.

Would the controls be wire routed meaning a long cable placed through the cargo area to the front of the vehicle or through an open window to the driver area? Is trailer mounted motor only for pushing, would it have reverse to backup? Why not add additional storage? Hard for me to get past the practicality questions to discuss only design.

But I'll try. I see a steering column mounted throttle control. If by yourself, how easy would it be to control the throttle while steering and braking the car? I sort of picture an old style thumb switch to control throttle clipped or mounted somewhere for easy access. Would it be drive by wire like a video game controller. On or off only, Pressed (on) and depressed (off), tapping would provide coast mode.

If your going to make why not make it dual fuel, e86 or cng?

Would a regular 2" tow hitch be able to take withstand the weight of a car? The hitch is designed to take the weight of a trailer and what its carrying. Wouldn't be adding the weight of the car to this as well?

If you are using this to restore charge can most electric vehicles be pushed started to capture the regenerative energy? Would it need to be in N or D/B?

How would the trailer be balanced? Two or four wheels?

my 1st (and last) thought is to have a very strong connection to the hitch since it's would be taking very different forces.. the rest of it is too clever for me. I'm watching with interest.

co2112 said:

What is the practical use of this? And how much would it cost instead of a tow? I am assuming this is the push the car when dead? Or is it to provide additional power for hills reducing the need to drain you battery?
Is this even street legal? Is a tow hitch the best connection for this?

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Those are ALL outside of the parameters of this thread. It doesn't have to be practical, cheaper than a tow, restricted to pushing the car only when it's dead, etc. Street legal is important, so I'll just stipulate that above (and CARB - EPA compliant).

Would the controls be wire routed meaning a long cable placed through the cargo area to the front of the vehicle or through an open window to the driver area? Is trailer mounted motor only for pushing, would it have reverse to backup? Why not add additional storage? Hard for me to get past the practicality questions to discuss only design.

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Let's assume that the control cable will pass through into the car with whatever the tow vehicle has available... hole in the trunk, window, etc. Obviously, a dedicated car could be properly and safely wired up, but that is specific to the tow vehicle, and I'm not interested in discussing the 1000 ways to do this simple task.

I guess it could have reverse, and in the situation of a Bug motor, a reverse gear would be available. I can't think of a need for it, however. Let's assume that this strictly pushes. Yes, it could have additional storage.

I see a steering column mounted throttle control. If by yourself, how easy would it be to control the throttle while steering and braking the car? I sort of picture an old style thumb switch to control throttle clipped or mounted somewhere for easy access. Would it be drive by wire like a video game controller. On or off only, Pressed (on) and depressed (off), tapping would provide coast mode.

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If this is steering wheel mounted throttle control, it would almost have to be wireless to the CCB. This steering wheel throttle control would be no different than riding a motorcycle. Right hand throttle (maybe a thumb control, like an ATV, or pull like a motorcycle brake lever) while the driver also has a firm grasp of the steering wheel, left hand on the steering wheel, feet fully available for brakes.

There is too much power available, and too many variables with tow vehicles to make it ON/OFF power.

For safety and probably for legal use, the throttle control needs to default to the idle positiion

If your going to make why not make it dual fuel, e86 or cng?

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It could be any of those. It's not pertinent to the design of a control logic.

Would a regular 2" tow hitch be able to take withstand the weight of a car? The hitch is designed to take the weight of a trailer and what its carrying. Wouldn't be adding the weight of the car to this as well?

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The trailer tongue weight won't change. For our discussion, the trailer hitch is perfectly adequate.

If you are using this to restore charge can most electric vehicles be pushed started to capture the regenerative energy? Would it need to be in N or D/B?

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The tow vehicle (assuming it's an EV... it could be a gasoline car) could be in ANY drive mode "gear" (except reverse?), and yes, it would completely charge an EV with regen, as well as propel the car as far as the fossil fuels keep getting replenished.

How would the trailer be balanced? Two or four wheels?

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Two wheels.

For a pusher trailer, how is the required horizontal rating of the hitch connection determined? E.g. is the requisite "tow" weight equal to the weight of the "tow" vehicle?

In the context of the Nissan Leaf as the "tow" vehicle, can one get a hitch for the Leaf rated at 4000 lbs "tow" weight?

Cheers, Wayne

Does it need to have two wheels? We considered using the back half of a motorcycle on a hinged mount that would only be lowered on the highway.

LTLFTcomposite said:

Does it need to have two wheels? We considered using the back half of a motorcycle on a hinged mount that would only be lowered on the highway.

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Ok, one or two wheels. Again, wheel count doesn't have any affect on logic to safely control this.

wwhitney said:

For a pusher trailer, how is the required horizontal rating of the hitch connection determined? E.g. is the requisite "tow" weight equal to the weight of the "tow" vehicle?

In the context of the Nissan Leaf as the "tow" vehicle, can one get a hitch for the Leaf rated at 4000 lbs "tow" weight?

Cheers, Wayne

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Tow ratings have a number of variables that just flat-out do not pertain here:

1) First, BRAKING is the biggest variable. You can bet that whatever rating is tested, it was determined safe to stop (at least once!). This may require "tow modes", bigger brakes, brake cooling systems, etc. Imagine going down a big hill with a LEAF with a cold, fully charged battery (no reg available) with 4000 pounds behind you (more than the actual weight of the LEAF). Good luck. If the brakes hold out, they will be cherry red and burnt out at the bottom.

2) Cooling of the drivetrain. For gasoline / diesel engines, this is primarily larger radiators and bigger coolant pumps

3) Weight of the vehicle. You don't want the trailer controlling the tow vehicle! A heavy enough trailer might push you off the road, or cause you to lose control.

4) Tongue weight. This is usually 10% of the trailer gross vehicle weight.

wwhitney said:

For a pusher trailer, how is the required horizontal rating of the hitch connection determined? E.g. is the requisite "tow" weight equal to the weight of the "tow" vehicle?

In the context of the Nissan Leaf as the "tow" vehicle, can one get a hitch for the Leaf rated at 4000 lbs "tow" weight?

Cheers, Wayne

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You are not trying to pick the whole car up by the hitch. Just trying to provide a steady 20kW push from the trailer so you can extend your range to go on a cross country trip and be able to make it between charging opportunities.

TonyWilliams said:

Tow ratings have a number of variables that just flat-out do not pertain here: . . .

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Thanks for enumerating those issues. I agree that these factors don't come into play with a push trailer, I guess I didn't phrase my question very well. What I was trying to say is that the horizontal force the hitch connection is going to have to transfer is comparable to the case of towing a loaded LEAF, i.e. towing a load of 4,000 lbs. How strong is the 2" hitch connection, for horizontal forces?

Sendler2112 suggested that the power requirements for maintaining freeway speed would be 20kW, which sounds plausible. Say that's at a speed of 60 mph (26.8 m/s). Since P = F dot v, the calculated horizontal force on the trailer/vehicle connection is 746 newtons, or only 167 pounds force. I guess the strength of the hitch connection is a non-issue for steady state freeway speed.

Now how about during acceleration under pusher power only? The force required will be much bigger.

Cheers, Wayne

wwhitney said:

Now how about during acceleration under pusher power only? The force required will be much bigger.

Cheers, Wayne

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As you calculated, we aren't working with enough weight, acceleration or power to be a factor. 2" receiver hitches are grossly overbuilt. Have you ever heard of one failing?

TonyWilliams said:

As you calculated, we aren't working with enough weight, acceleration or power to be a factor. 2" receiver hitches are grossly overbuilt. Have you ever heard of one failing?

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I know nothing about them, I'm just curious about the physics of the arrangement.

Will there be conditions where the hitch connection has to transfer a significant moment about the vertical axis? It seems like designing a rear wheel drive car, where the drive wheels, instead of being connected to the front of the vehicle via the approximately 5' (?) wide frame are connected via a 2" wide steel tube. So the force couple related to any moment will be amplified by a factor of 30 or so.

Also, speaking of rear wheel drive, for a front wheel drive "pushed" car like the LEAF, will effectively changing the car from front wheel to rear wheel drive create any steering or control issues? Are the steering and other control systems of a FWD car tuned differently from a RWD car?

Cheers, Wayne

Safety first / human factors:


1) I think I would add a separate switch to the throttle control unit that is enabled when pulled past the idle position (with a simple but strong spring) that should the throttle not respond to reduced throttle opening (won't go to idle, or won't respond), then increased pulling will hit a kill switch that will:
a. Close FSOV
b. Kill power to throttle actuator (springs will manually close throttle to idle)
c. Kill power to ignition coil
d. Kill power to clutch control (which will manually move to the disengaged position)
e. Arm any fire bottle squib (fire suppression... will light up FIRE switch to be pushed if a fire is suspected)
f. May lock up the electric brakes?

Somebody who panics a bit (or has serious safety concerns) when the power doesn't reduce will NATURALLY pull harder on the throttle. Blowing the fire bottle(s) takes a conscience act, and is only armed with the throttle pulled to the PANIC position.

2) Loss of electrical power will do all the above EXCEPT lock up the brakes

3) If the Cockpit Control Box should become unsecured, fall, damaged, the vehicle brakes can accomplish all the above, up to and including operation of the trailer brakes

4) There probably needs to be some logic that 12 volt from the vehicle brakes is available... not sure now. Maybe a circuit of trailer light power? That would mean that trailer lights would always be on to enable the trailer power.

5)

wwhitney said:

Now how about during acceleration under pusher power only? The force required will be much bigger.

Cheers, Wayne

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The pusher will not have the performance to accelerate the leaf like a 100kW motor would. That is up to the Leaf. In other words, the 30kW bug engine would fire up and run wide open on the highway near it's peak efficiency to just maintain 70 mph plus maybe a little bit of regen. Going up a hill the Leaf accelerator is applied to add power. Going back down the hill gives back more regen. If the battery is near 100% you shut off the pusher and put it into neutral. Re engage as needed for the required range of the day.

For reverse. I was thinking of being stranded with one occupant. It would help even in the event you are stranded in a flat parking lot without the ability to pull through.

co2112 said:

For reverse. I was thinking of being stranded with one occupant. It would help even in the event you are stranded in a flat parking lot without the ability to pull through.

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Obviously, reverse is available. I would specifically eliminate 1, 2 and 3 gears and maybe put a padlock on Reverse, since putting the thing in the wrong gear at the wrong speed might blow up the motor.

It doesn't really need a linear actuator on the gear shift, since you can tow it in neutral, and when you want pusher capability, stop and manually put it in 4th gear (or reverse). If you put it in gear without powering up the trailer electrics, the clutch will be engaged, so there needs to be some logic to put the thing in gear manually or with automation, it will power up the unit and disengage the clutch.

Making a mode to do this in reverse sounds like a real pain in the *ss. Actually, the more I think about, the less I like the idea.

So, traveling at 30-40mph (the calculated 4th gear idle speed), the trailer in 4th gear and electically powered up, so start the motor. I guess you could start it at the same time you put it in gear, since it will stall if the clutch accidentally engages (little danger), and it won't start if the clutch is engaged while in gear. Keep in mind that this is FAR more dangerous in reverse, or a first or second gear. There's enough torque for this to get away from you and hurt somebody.

It can have a manual clutch engage button on the CCB (since an automated clutch engagement is far from simple), and when at the perfect speed with all the "needles in the green", engage the clutch and push up the Bug power while reducing the the EV power to idle.

There is a danger that the clutch could disengage under load, and overspeed the Bug motor, so there needs to be an interconnect between the throttle and clutch so that the throttle is at idle any time the clutch is disengaged.

So, a manually actuated clutch from the CCB.

Pressing the brake should only close the throttle manually. Maybe the clutch does need to disengage to, since it's interconnected.

Lots of thinking out loud.

A centrifugal clutch would simplify the control logic, but I don't know if an off-the-shelf unit with appropriate horsepower and RPM ratings (and physical size) is available.

There are sprag clutches that can hold 20kW of power and also freewheel at highway speeds. A sprag clutch is a one-way clutch so as soon as the engine turns off there would be no need for a transmission to disengage. The only problem I see with a sprag clutch is that the engine will be forced to turn backwards if driven in reverse. I guess the bright side is that we don't tend to drive in reverse for very long, so it might not make much difference.

The sprag clutch could be part of a chain drive from a V-twin engine (may I recommend a Kohler EFI Command Pro propane engine) to the wheel on a one wheeled trailer. The trailer could be made to pivot off of a U-joint at the trailer hitch instead of a typical ball. This would keep the single wheeled trailer from falling over and still have full articulating motion.

I don't see why the engine couldn't be controlled by a simple on-off, especially with a one-way sprag clutch. You start the engine when you need constant cruising power. The engine can be sized to give only a constant 20kW at 60mph (which would be about 15kW at 45mph and 10kW at 30mph.) Any slight inclines up or down can be aided by the power and regen of the Leaf. Since engine torque (force) tends to have a top limiting curve whereas aerodynamic drag increases exponentially simply slowing down would incur more regen and less power needed from the Leaf if you were losing too much power from the engine. Also, with the simple on-off approach, if the Leaf starts to get a full battery from regen control, the engine can simply be shut off at any time.

I'm not keen on a spray clutch (I don't think it would be durable), but there are auto clutches for motorcycles.

Rekluse and EFM are two.

Not cheap, and would take a LOT of machine work to integrate with a different motor.