Why do many hybrids have little or no trailer towing capacity?

Question

I have noticed that many hybrid vehicles have little or no trailer towing capacity. For example, the Toyota Auris Touring Sports hybrid model has 375 kg trailer towing capacity (unbraked + braked) whereas the non-hybrid model has 450 kg unbraked and much higher braked towing capacity. The Auris non-Touring-Sports model in the hybrid variant cannot tow a trailer, and Gen1-Gen3 Priuses also cannot tow a trailer. The Yaris hybrid model cannot tow a trailer, but the Yaris non-hybrid can tow 550 kg unbraked and much higher braked trailer.

Why is this the case? Considering that in Finland, the maximum speed allowed for trailer towing is 80 km/h, and that you can drive continuously for 120 km/h with no trailer, surely the load trailer towing puts on the engine, inverters and motor-generators is lower than the load when driving faster than permitted with a trailer. Similarly, hybrid vehicles can be used in a hilly terrain. Driving up a hill is heavier on the system than towing a trailer on a flat land.

Fortunately, this seems to be changing. The Gen4 Prius has 725 kg trailer capacity (braked + unbraked).

Why is the trailer towing capacity on many old hybrids low or even zero?

Why is this situation changing now?

Answer 1

Brake performance, or vehicle stability, not motive power are the primary concerns with trailer limits.

Why no trailers in early hybrids?

Several considerations; first is the battery. In hybrids with regenerative braking the electric motors act as generators producing electricity. Now that electricity has be put somewhere; it goes into the battery. But what if the battery becomes fully charged on a long downhill? Now were do the electrons go. The design has the system switch to the regular service brakes and stop regenerative braking. Now the power available in the brake system governs stopping performance.The extra mass of a trailer causes the switch to happen much sooner. Hybrid designs vary in service brake power and heat load capacity. When brakes overheat on a long downhill effectiveness falls of dramatically.

Why is the trailer towing capacity on many old hybrids low or even zero?
Possibly in the earlier era engineering concerns about braking and or stability won out of the marketing departments demands for features to advertise. Engineering was more concerned with proving the design than peripheral concerns such as tow capacity.

Why is this situation changing now? More experience with, and changes too, the hybrid design allows engineering to feel confident of their design.

Note: The design must be able to handle speeds higher than the highest allowed speed limit worldwide, plus and extra safety factor for emergencies. In my area maximum speed allowed it 60 mph (96 kph)

Answer 2

Lower towing capacity for Toyota hybrid models because:

1. Low weight of the car? NO. Hybrids could be even heavier due to the high voltage battery.
2. Possible overheating of engine? NO. Hybrids are more efficient so they don't need so much cooling.
3. Weak break system? NO. Hybrids have the same systems like regular car (friction breaks, engine break) and regenerative breaking as extra. Moreover, engine break works better because revs are electronically controlled and kept at the same level no matter what the speed is.
4. Weak clutch? NO. Older Toyota hybrids don't have any clutch. Prius Prime has one for EV/non EV driving.
5. Weak transmission? NO. Transmission strength is built according to front wheel friction which stays nearly the same.
6. Transmission cooling? YES. Older hybrids have passive cooling of the transmission. Due to heavy load at low speed the temperature raises which can cause failure of lubrication and damage to the transmission. Newer versions may have better (active) cooling of transmission. Highlander hybrid can tow the most in special version with active transmission cooling with extra cooler only.

Answer 3

There's a good article that came out after the most recent activity on this thread that describes how each car must be put through a "homologation" process that is an engineering test of the automobile's ability to tow a load. Basically it sounds like this is contrary to the car maker's best interests because at this point people who buy hybrids are only interested in getting the maximum mileage. Towing capacity is secondary and it appears engineering time is limited.

I don't buy the other arguments, except weight. As mentioned before, the brakes aren't different, and if the battery charges fully on a long downhill, the car will just switch to either engine braking, or mechanical/disc braking. Unless the hitch mount won't physically fit on the frame, I can only see the lack of incentive for homologation testing as a reasonable explanation.

I could see weight as a contributing factor here - if the car with a gas engine is rated to tow 500 lbs (probably with the assumption of a fully loaded cabin as well), and you add another 400 lbs of battery, maybe there isn't really an incentive to redo the testing to see if you can get that 100 lbs of towing capacity to be certified.