Possible smallest range extender engine

My understanding is you have to "hack" the code in the i3 to have it turn on the REx at higher SOC. By default it only comes on when the battery is in a low state of charge. bwilson4web mentioned it in his comment.

 

Once you code the North American BMW i3-REx to enable at 75%, the problem is solved.

Bob Wilson

 

No loss of range or speed until out of gas and charge. But once out of gas, use the battery to reach the nearest gas station. Better still, when in last 1/4 tank of gas, refuel before the gas runs out to preserve the SOC. If the gas station is in a dip and descending off-ramp, you can gain 1-2% SOC reaching the pump. Enabling REx before leaving the pump preserves the higher charge.

Bob Wilson

 

From many miles with both a genset trailer and a pusher trailer, I've decided that 15 kW of mechanical power is needed for indefinite highway cruising in a compact EV that has enough battery to supplement the engine up a mountain pass, and accept regen all the way down. I've taken trips of over 300 miles on a 'single charge' of my 16 kWh i-MiEV using an aircooled VW pusher engine. (Overkill and not very efficient, but a real rocket, with double the stock horsepower!) Using only a 5 kW generator increased the range of my 30-mile conversion by a minimum of 50% in mixed driving. Leaving the genset running at my destination would of course enable a return, but neither pushing nor generating produced a hybrid of much better than 20 mpg.

 

When you keep getting overtaken on a highway going up a mountain, when you keep getting passed by cars and trucks that honk at you because you're driving dangerously slow -- doubly dangerous when it's on a twisty mountain highway with limited viewing distance -- and when you have to keep pulling over to get out of the way of other vehicles... then it's definitely a problem.

But obviously it depends on the road. If you're on a local mountain road, the speed limit may be as low as 25 MPH anyway, and if so you'll be fine. But if you're actually on a highway, and heavy trucks are passing you despite not being able to see far ahead on twisty roads, or honking at you to get out of the way... that can get pretty hairy.

See lots of discussion here: "US i3 Rex dangerous when climbing hills"

Of course, you can say "Well, just don't drive such a car on such roads past the point where it starts to slow down because the battery pack is exhausted. Stop and recharge before that happens."

Perhaps there will be some drivers who will plan ahead that well. But obviously not everyone does. If they did, nobody would ever run out of gas, and that takes a lot less planning than driving a plug-in EV past its normal range.

 

Unfortunately that is not a clarification, it's just confusing the matter.

What the BMW i3's range extender actually does is turn the car into a serial hybrid, with the engine powering a generator which mostly feeds electric power directly to the motor, bypassing the battery pack. I believe it does, when necessary, and under normal circumstances (that is, when not accelerating strongly or when climbing a hill), divert some of the power to keep the battery pack at some minimum state of charge (is it 30%?), but most of the power doesn't go into or thru the battery pack, and most of the time it's not feeding any power to the battery pack at all.

 

That is a good clarification to point out that the REx powers a generator, which can feed energy to both the electric motor as well as the battery. The clarification I was trying to make was that the ICE doesn't have a transmission to drive the wheels directly; rather all motive force is through the electric motor, with both the ICE and battery pack as source of energy for that electric motor.

 

I've seen reports about the prototype and plans but no official release. Google didn't find an official release either. Source?

My reason for asking is my BMW i3-REx has a two-cylinder, water cooled, 34 hp (25 kW), 647 cc motorcycle engine. This has just enough power to sustain 70 mph. A Wankel sounds like a good fix weighing less and smaller but only Mazda seems to have them.

Thanks,
Bob Wilson

 

Wankel engines are smaller and lighter than ordinary piston ICEngines, given a comparable horsepower rating?

If so, then it seems odd they aren't used more. Do they have a higher cost, or lower lifespan?

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Their major problems from what I've read:

1. Low thermodynamic efficiency - primarily because they don't expand the combustion gas enough. Speculation, it may be mitigated by delaying the intake port close.
2. Full loss oil - typically inject or mixed, the oil goes out the exhaust and that complicates emissions. However, there are reports of alternate oil ports that can reduce, not eliminate the amount lost.
3. Spark leakage - the round hole leaks between charges when the seals pass. Focused laser ignition with a diamond surface, optical window might work. There may be other approaches that 'spit fire' into the combustion chamber.

Bob Wilson

 

Measuring the displacement of a Wankel rotary engine is not straightforward. I think they had challenges with the seals, and with emissions?

 

The engineers at GM, Honda etc (exc BMW) have already figured out the correct size ICE to generate adequate power to maintain a safe speed under all terrain conditions. IMO, the I3 Rex is unsafe and crippled by an underpowered ICE generator. There was a class action lawsuit launched against them back in 2016: https://www.carcomplaints.com/news/2017/bmw-i3-rex-lawsuit-range-extender-loss-power.shtml

 

This is inaccurate as we have a 2014 BMW i3-REx:

• 463 mi delivery drive home from Charlotte NC to Huntsville AL over the I-40 Asheville NC to Knoxville TN pass
• 1,400 mi round trip from Huntsville AL to Stillwater OK
• 280 mi round trip Huntsville Al to Spring City TN to watch eclipse

The car sustains charge at 70 mph without a problem. The battery has plenty of capacity to handle passing and hill climbing. Descending hills brings the battery back to the sustained SOC. Reducing speed to 65 mph or following traffic also lets the REx bring the battery back up. Regeneration pulling off the highway can add 1-2% over the original SOC leading to bumping up the SOC on long trips.

Bob Wilson

 

My brother has an i3 REx, and he tows a small trailer on a camping and canoe trip each year, and up in the White Mountains in NH, it will slow down to 50-55MPH up very low climbs, with the trailer in particular. But it is hardly a safety issue.

 

The White Mountains are part of the Appalachian mountains. That is an old and fairly worn-down mountain range. The average slope in the Appalachians is going to be far more gentle than the average slope in the Rocky Mountains.

How steep the slope is matters a great deal when you're depending on the power from what is only a two-cylinder scooter motor, which is what the i3 REx uses as a range extender!

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