Prime vs. Hyundai Ioniq

Frankly, Hyundai has never been known for their steering and handling. We own a Sonata and Elantra GT and the steering characteristics are pretty awful overall. Heck, I think our 2010 Prius feels better than either of them.

 

The claim was that it would be more efficient because of the larger diameter motor - that is not true.

The Ioniq has a 32kW motor so it is lower power than the Prius MG2 @ 53kW.

I haven't seen a figure for the battery power on the Ioniq - I would assume it is also 32kW.

I also haven't seen a battery power figure quoted for the gen 4 Prius. The two motor generators are rated at 53kW and 23kW - the battery power is probably the difference between the two i.e. 30kW.

From the figures for specific power they give in an SAE paper it would max out as ~44kW but I expect it is much less than that.

kevin

 

I thought the claim here was simply that the Ioniq motor produces more torque. It does, 125 to 120 foot pounds.

Hyundai claims the drive train is the most efficient in torque-transfer. 2017 Hyundai Ioniq Hybrid and EV First Test Review

The Ioniq battery is 1.56kWh and 240 volts. The Prius Li-ion one is 0.75kWh and 207.2 volts.

 

The motor may produce more torque but it's power is limited to 32kW. Power is the product of torque and RPM.

The actual torque figure is pretty meaningless - MG2 on the gen 2 Prius had 295ft-lbs of torque but the gen 4 has about 120 ft-lbs(163nm) only marginally less than the Ioniq. Does that mean the gen 2 had better performance than the gen4 or the Ioniq? Of course not, in part because battery power was limited to 20kW in gen 2.

One big advantage for the Ioniq in low speed traction is that the electric motor is followed by the variable ratio gearbox (similar to the Honda Hybrids with the IMA system) that allows better acceleration from rest or for hill climbing.

The "torque-transfer" claim is very vague - most mechanical transmissions are expected to be about 95% efficient - Toyota states they improved the mechanical loss by 22% in the gen 4 - but don't say what from. By the time you get to values such as 95% the effect of any improvements is limited.

kevin

 

The gen2 didn't have a reduction gear for M/G2, so it needed a more powerful motor. Power-split hybrids have a series hybrid path that allows the traction motor to exceed the battery power limit. This has a cost in requiring a more powerful M/G to provide that extra power, and a constant split of ICE torque going to that M/G.

The high low end torque also means the transmission can also implement shallower gear ratios or allow upshifting at lower speeds.

When I first heard the torque-transfer efficiency claim, I was wondering how the torque split of a power-split hybrid would be accounted for.

 

"The gen2 didn't have a reduction gear for M/G2, so it needed a more powerful motor."

Precisely - you cannot arbitrarily compare torques of the components without taking into account the gearing - the torque of gen 4 MG2 is comparable to that of the Ioniq and yet it has a ~3:1 gear reduction before it is combined in the final drive (another 3.5:1 reduction).

The torque at the axle for the Prius from the electric motor is therefore 120*3.117*3.476 = 1300ft-lbs.(below about 25mph)

For the Ioniq however the stepped gearing allows 1st gear to provide a fairly impressive 125*3.867*4.188 = 2024 ft-lbs at the axle. This is probably only up to 10mph or so before the motor hits it's power limit (I haven't calculated that yet)

In 6th gear the maximum torque would reduce to a more modest 125*3.045 = 380 ft-lbs.

"The high low end torque also means the transmission can also implement shallower gear ratios or allow upshifting at lower speeds."

It is still power limited - I see that the maximum battery power on the Ioniq is specified at 42kW even though the motor is specified at 32kW.

The gear change points will be pretty much determined by the BSFC curve of the engine - the eCVT on the Prius can always allow the engine to be at the optimum RPM for the power requested by the driver - the Ioniq is a bit more limited because it has stepped gears but the extra battery capacity and power can probably smooth over the discontinuities.

kevin

 

There were no hydro in the 2001-03 steering. It was pure electric which led to some interesting problems with dirty, torque encoders.

Bob Wilson

 

A step transmission can have better overdrive ratios than a CVT, and give the car better cruising efficiency at steady speeds.

 

Actually I disagree.

For the ioniq the final drive ratio in 6th gear is 3.045. The gearbox ratio 0.767 and the tires are 195/65/R15 (same as Prius) with 832 revolutions per mile.

At 60mph this gives us an engine speed of 832*3.045*0.767 = 1943 RPM. This is about 30mph/1000RPM engine speed.

In the Prius I usually see engine speeds in the 1500-1700RPM range at 60mph - ie more overdrive than the Ioniq.

The mechanical gearing of the Ioniq can give a better efficiency than the Prius eCVT if significant amounts of power have to go through the electrical path which only has ~80% efficiency compared with the mechanical path at ~95%.

When you get to the sorts of overdrive that both the Prius and the Ioniq have, more overdrive is not necessarily better. It is far more important to be at the correct gearing.

The advantage of the eCVT in the Prius is that since it has a continuous range of ratios to choose from it can always be at the correct value to provide the optimum fuel efficiency. With its fixed steps the Ioniq will often have to choose between two gears neither of which is perfect. Obviously to get the fuel consumption figures that they get they have done a good job and the efficiency advantage of mechanical vs electrical transmission may help more than not being able to get exactly the right gearing.

In this Prius fuel consumption map the blue line shows the operating line that the control system will attempt to follow to achieve the best efficiency.

The engine output power and RPM are monotonically increasing from left to right along the operating line.

From the driver's throttle setting and the road load a specific amount of power will be demanded. The engine control system will select the operating point that matches that demand - there is only one point on the curve that will meet the requirement - too far to the left and the engine will not output enough power, too far to the right and there will be an excess.

The hybrid battery will either supply or store any short term differences in power between what is required and what the engine outputs but in the long term the engine must supply the correct amount of average power or the hybrid battery will discharge or overcharge.

kevin

 

My nickname for my Gen 1 was "the bobsled". Actually I think a bobsled probably has better steering feel than Gen 1 did. My Gen 2 is a world of difference. I have driven (but not owned) Gen 3 and Gen 4 and it keeps getting better with each generation..

 

I wasn't thinking of the Ioniq specifically, but of the fact that 8 to 10 speeds are starting to become the norm. Not only do they give more ratios to select from, but also can contain overdrive gears for the 70mph and higher speeds. Which may not be achieved on most official tests, but many drive at while on freeways and interstates.

Does Toyota make the high and low end ratios of their eCVTs readily available?

 

The Prime got rid of the steering stiction of the Gen-3 and makes driving a lot more fun.

I've had our Gen-1, Gen-3 and Prime from 0 mph (0 kph) to 100 mph (160 kph) without a problem.

Bob Wilson

 

I have an Ultragauge on my gen4 Prius and I can see the engine speed operating down to 1200-1400 or so RPM will driving at high speed. I also have the load factor displayed (an indication that the engine is doing work) and it is usually at 95% showing that it is working hard at that time.

With the gen4 Prius intermittent operation can occur up to 68mph showing that there is enough range of control to make the engine RPM go to zero if needed.

I don't think any conventional cars would have a high a gearing as the Prius can obtain because they always need to retain acceptable drivability - this equates to spare torque capability so that when the driver wishes to speed up, even slightly, or the road-load increases there is some ability to do so without having to change gear.

Until recently I had a Mazda 3 which although it only had 6 gears its 6th gear was tall with about 32mph/1000RPM. On level ground at 60mph the engine would run at just below 2000RPM (similar to the Ioniq) but at a load level of 50-60% - so it had about 50% torque capability remaining without changing gear. Even then there were some overpasses that annoyed me because it would prompt the transmission to change down when it encountered the slight hill (I would often put it into manual to prevent that even though the car would slow down a bit!).

kevin

 

this is why I said you guys know the prius I know the ioniq, makes comparing so much easier

 

Up to 115 mph so far, no problem; had plenty left over!!

.

 

the ioniq 3 variants have the following

	model battery	kWh	Ahr	volts	motor kW	motor PS
1	hybrid	1.59	6.5	240	32	43.5
2	plugin	8.9			44.5	61
3	EV	28	78	360	88	120

 

The mechanical complexity of a transmission with that many gears makes me cringe.

 

Not directly but the limits are determined by the maximum rotation rate of MG1. With the gen4 the engine can be stationery up to a road speed of 68mph - does that mean the ratio is infinite?

The other end of the eCVT ratio is also not really meaningful as the car can be stationery with the engine rotating so is the ratio infinite on that end as well?

As I mentioned in a previous post there is more overdrive than is needed so then it becomes what gearing is desirable - the Prius can set the engine speed to optimize efficiency pretty much independent of the road speed. The efficiency of the system does reduce the further away the eCVT differs from the mechanical ratio that occurs when MG1 is stationery. (I haven't worked out what that ratio is for the gen4 Prius PSD).

With conventional vehicles the important item is not just getting a high overdrive ratio but also maintaining drivability. if the gearing is too tall gear gear changes will be required with even slight changes in throttle or road incline - and is considered undesirable if it occurs too much. It usually means that the engine only runs at 60-80% of its capability so there is reserve available for those scenarios.

A hybrid (either eCVT or with stepped gearing type like the Ioniq or Civic) is in a better position because the hybrid battery can provide that reserve so the engine may run at 90% of its capability.

A hybrid requires a small amount of reserve torque to be able to spin the engine faster when changing gear (or the equivalent on the eCVT).

kevin

 

And the increasing complexity of intake, valve and injection control doesn't?

Nothing on a car with an ICE is 'simple'. The HSD might be mechanically simple, but the software isn't, and an issue with the software lead to a large recall of the gen3 because is lead to cars shutting down on the road. There were inverter issues on some Gen2s, which is something a traditional ICE car has to worry about.

Anecdotally, the only post 2000 car with transmission issues that I know about was my brother's Accord. In that case, Honda used a transmission that wasn't robust enough to handle the V6's output. Plus, the car had been through a flood up to the windows.

Cars have far better reliability now than they did in the past. Cars staying on the road for too long is actually a problem for the ICE car companies now.

 

I appreciate the simplicity of the eCVT. Adding all these gears is the wrong approach. Software doesn't wear out.