REX Engine kWh output

Does anyone know what electric power the REX generates? On the Chevy Volt the ICE makes more than driving power.

hambrook said:

Does anyone know what electric power the REX generates? On the Chevy Volt the ICE makes more than driving power.

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It has 34hp and will provide a supply of up to 25kW to the battery.

... and careful with the units, hambrook. You wrote "REX Engine kWh output", when it was kW you were looking for. It's a common mistake.

And welcome to the forum

ChrisC said:

... and careful with the units, hambrook. You wrote "REX Engine kWh output", when it was kW you were looking for. It's a common mistake.

And welcome to the forum

Click to expand...

I was looking at the kWh figure. 25 kWh seems low as my LEAF currently has an 80 kWh motor so 25 kWh would struggle to get it up a hill....

Hi Grant,

I think what Chris means to say is that you want to look at the kW figure and not at the kWh rating if you want to know something about the cars power output.
kW = energy delivered at a given moment (i3 electric motor 125 kW peak / 75 kW nominal, REx engine 25kW)
kWh = energy delivered over time (1 kWh = 1 kW delivered during 1 hour = 3,6 MJ (megajoule) of energy)

So, for the REx, if it delivers 25kW (*) and it runs for an hour, it has delivered 25kWh.
Keep in mind the REx works in combination with 20% charge remaining in the battery. The battery will supply the electric motor with all the power it needs at any given moment. The REx tops up the battery when power demand < 25kW.

Greetings, Steven

(*) PS: not to doubt Toms expertise, but as far as I know the engine itself is rated 25kW. I expect this means that generator and charging losses have to be taken into account, so I expect that around 20kW actually reaches the battery, but do correct me if I'm wrong here.

hambrook said:

ChrisC said:

... and careful with the units, hambrook. You wrote "REX Engine kWh output", when it was kW you were looking for. It's a common mistake.

And welcome to the forum

Click to expand...

I was looking at the kWh figure. 25 kWh seems low as my LEAF currently has an 80 kWh motor so 25 kWh would struggle to get it up a hill....

Click to expand...

But does the Leaf or the i3 consume 80 kW *continuously*? I'd imagine that the energy consumption varies with acceleration, deceleration, hills, speed etc.

The REx will sit churning out 25 kW continuously - pushing, say, 20 kW into the battery. So long as the *average* consumption is less than the amount going in continuously then the car's not going to struggle.

Thanks Stevei3 for the additional info. That is indeed what I was talking about.

Hambrook, be careful to use the correct units -- kW for engine power, kWh for battery storage.

kW and kWh are very much different, in the same way that speed and distance are very much different!

Stevei3 said:

Hi Grant,

I think what Chris means to say is that you want to look at the kW figure and not at the kWh rating if you want to know something about the cars power output.
kW = energy delivered at a given moment (i3 electric motor 125 kW peak / 75 kW nominal, REx engine 25kW)
kWh = energy delivered over time (1 kWh = 1 kW delivered during 1 hour = 3,6 MJ (megajoule) of energy)

So, for the REx, if it delivers 25kW (*) and it runs for an hour, it has delivered 25kWh.
Keep in mind the REx works in combination with 20% charge remaining in the battery. The battery will supply the electric motor with all the power it needs at any given moment. The REx tops up the battery when power demand < 25kW.

Greetings, Steven

(*) PS: not to doubt Toms expertise, but as far as I know the engine itself is rated 25kW. I expect this means that generator and charging losses have to be taken into account, so I expect that around 20kW actually reaches the battery, but do correct me if I'm wrong here.

Click to expand...

Feel free to question me Steven! You are probably right and it supplies slightly less than the output rating. I will be at the LA Auto show for the i3 press driving event and interviews. I'll try to get that answered but the exact supply may be something they don't reveal. I'll see if I can get clarity on this though.

Just to add my (newbie) pennyworth, 25kW is the same as 34hp (33.5 to be more exact), but in SI units. I agree that the generator output is likely to be lower, nearer 19kW.

Cars almost always have an installed power rating that is several times the power required to cruise on a level road, so as to allow for acceleration and hill climbing. The i3 is no exception with a 125 kW (≈170hp) motor. The average demand is likely to be much nearer to 19kW at 70mph on the flat. The generator only has to meet the average demand to reduce battery depletion to a low level.

The battery capacity (stored energy kWh) and the motor power (kW) of the i3 are sufficient to take it over a very considerable climb at a sensible speed. This would deplete the battery because the (say) 19kW generator power wouldn't meet the extra demand for climbing, but part of the energy should be recovered with motor braking on the descent.

I've asked BMW UK for more on the generator output and the highest steady speed on the level without battery depletion. No answer on that yet, but it appears that the REx dashboard will show when depletion is taking place, so anyone taking a test drive in good conditions should be able to tell us.

was just trying to figure out the kw output of the Rex motor by going to a couple of the bmw 650 scooter reviews.

Anyone know if the engine is detuned? This review below states that the engine is capable of 44 kW - however that may be at a very high engine speed of 7000 rpm or so. Perhaps they lower the engine speed to get down to 25 kW?

I also saw somewhere else that the engine was made by Kymco - anyone know if this engine is a BMW or Kymco engine?

http://www.topspeed.com/motorcycles/motorcycle-reviews/bmw/2013-bmw-c-650-gt-ar140197.html
"One of the newest addition in BMW’s scooters lineup is the 2013 C 650 GT Maxi-Scooter which is powered by a 44 kW (60 hp) engine. The scooter was designed to be as comfortable as possible and is aimed at those who are searching for an efficient and fast way of urban transportation. Talking about efficiency, the BMW C 650 GT has a fuel consumption of 5.6 l calculated at a constant speed of 75 mph."

Hi,

it is detuned, adapted, altered, you name it. It is rated 34 hp/25 kW by BMW. It seems like it is capable of delivering around 22 kW of electrical power for our car, though we need more km to be absolutely sure of this.

Steven

ok, chiming in here. I do not have any inside scoop on this engine and generator. But I do have some experience with engine ratings and outputs, so I'll add a bit.

Typically, manufacturers rate their engines based on peak horsepower. That's the number that grabs media attention. But from a practical point of view, engines will not last very long if they are run continuously at peak horsepower.

So, engines that are set up for stationary duty (the BMW generator is a typical stationary duty situation, which allows for the highest design efficiency) are derated for a continuous horsepower output. Typically 70% to 80% continuous duty horsepower is standard. While 25kw/34hp may be the engine peak output, 10 mins would be a standard limitation to preserve engine longevity.

Thus, I would expect somewhere about 20kw/27bhp as a continuous output. Factor in a generator of about 95% efficiency, and 19kw is reasonable continuous output for the generator. That's also probably the best operating condition (most efficient).

Another rule of thumb here...about 15 brake horsepower/hour for a gallon of gasoline. If the fuel tank is 2.4 gallons, expect that the generator will be able to supply about 25kWh worth of energy from that size tank. So if the driving conditions are easy and the electric motor can convert at about 4 miles to a kWh, it will result in about 100 miles of additional range...if conditions are closer to 3 miles per kWh, that's about 75 miles of range. Fuel consumption is thus about 27bhp/15bhp = 1.8 gallons per hour. I would expect that the tank will be empty in less than 1.5 hours (but depending on driving, battery storage may make it possible for 2-3 hours if there is a bunch of city driving).

John

Hi John,

a small correction regarding your data: the base enige is rated 60hp @ 7500 rpm. It is detuned from this, with EURO6 standards and longevety in mind, for use as 'stationary enigne' delivering 34hp. I would guess by the noise our REx makes, although I am not a two-cylinder expert, that it 'sort of' runs at the same revs a bike would make @ 120km/h (75 mph), so I would say ~4500-5000 rpm. Hard data for this is, regrettably, not available.
Our real world usage data (for the last two weeks) suggests that the REx can sustain the state of charge when using around 22kWh/100 km (62 mi) ) to propel our car at around 120 km/h (75 mph).

Steven

that does make a difference.

I think I'll be a "purist" on this car and stay away from the ICE. The idea of no engine maintenance, etc., after so many decades of fussing, is simply too attractive. I'm a farmer with lots of machinery, from a diesel tractor to electric golf carts and everything in between. It will be a welcome break to be able to keep the hand cleaner in the container with this one. I have to say that all the maintenance on our '99 BMW 323i has certainly been overly tiresome.

John