Last post on Jun 17, 2008 at 4:30 AM
You are in the Toyota Sienna
What is this discussion about?
Toyota Sienna, Hybrid Cars, Van
#5 of 44 Re: 2008 Sienna Hybrid - Do you want 4 cylinders or 6? [conger14]
Jun 26, 2007 (6:12 am)
Well, Toyota's secret is that they don't use an AWD drivetrain on hybrids, they are all FWD based. There is no rear driveshaft on the Highlander hybrid, for instance.
What they do instead is have electric-only assist on the rear axle.
That eliminates a rear driveshaft and a rear differential, so the weight penalty is much smaller.
The catch is it's not full-time AWD. It has a temporary electric-assist on the rear axle if the battery has enough charge.
#6 of 44 Re: 2008 Sienna Hybrid - Do you want 4 cylinders or 6? [msindallas]
Jul 03, 2007 (5:20 am)
I vote for the 4-cyl internal combustion engine for the Sienna hybrid.
It makes a lot of sense to not mindlessly buy a hybrid without calculating the increased cost vs. the fuel savings. But some detailed analyses conclude that despite its higher purchase price a hybrid may in fact be cheaper overall for a large fraction of drivers.
The least expensive power would be from a conventional internal combustion engine of the smallest size, but in the US we demand high acceleration.
It is a misconception that a hybrid has no advantage on long stretches of freeway or on long grades. It does. Another version of this misconception is that a hybrid is only an advantage in urban traffic where the regenerative braking operates. The regenerative braking contributes only a small part to the higher efficiency of the hybrid. Regenerative braking only recovers about 20% of the kinetic energy of motion.
The big V-6 is way overpowered for long stretches of level freeway travel at legal speeds. It takes say 50 hp (or maybe less) to push a Sienna down a level highway at 70 mph. This would be more efficiently done by a smaller internal combustion (IC) engine than a larger one. Also the IC engines in hybrids are set up to operate in a very efficient part of the operating curve. That is, the engine controls are set-up to not employ the IC engine at low power demand, where it is relatively inefficient.
If you had a hybrid, there would be no reason to rent a vehicle for highway trips, even those with long grades.
The situation where the big V-6 would be needed would be 2-lane mountain roads in Arkansas or in the Appalachians where you might want to quickly pass slow logging trucks.
#7 of 44 Re: 2008 Sienna Hybrid - Do you want 4 cylinders or 6? [jim314]
Jul 05, 2007 (7:41 am)
A big engine can be geared taller than a small one, though. At 55mph my 3.5l is barely humming along at 1600 rpm.
The 2.4l 4 cylinder could operate, say, at 4000rpm, where it makes its peak torque, via a CVT, but you'll end up using more fuel that way.
In other words you have 31% less displacement but you have to increase revs by less than 31% to end up using less fuel. 31% more revs than the 1600rpm my engine uses is just 2100 rpm or so. Can the 4 cylinder pull all that weight at such low revs?
I'm not convinced the 2.4l can effectively pull 4300 lbs, plus the full payload, plus the extra weight of the batteries.
I'd love to be proven wrong, though.
#8 of 44 Re: 2008 Sienna Hybrid - Do you want 4 cylinders or 6? [ateixeira]
Jul 05, 2007 (11:50 am)
When the operator is directing a vehicle with a CVT to cruise at a constant 55 mph the engine controls do not have the engine at 4000 rpm at the peak torque point. That would be equivalent to putting the 3.5L/auto tranny into a lower gear (like 3rd of five)at 55 mph, or driving a car with a 5-spd manual tranny at 55 mph in 3rd or 4th gear. The engine controls for a CVT are programmed to achieve the same sort of drivability as with a conventional automatic.
Driving at high power levels at constant speed in a low gear is done in racing where one needs instanteous acceleration or deceleration with slight movements of the pedal, but is ordinarily not wanted on the street because it is such a fuel waster. What happens in that case is that the waste heat going out the radiator and the tailpipe both increase dramatically, but the power at the wheels stays the same.
Section A-6 of this page from a NASA analysis shows a graph (Fig A-1) of engine efficiency vs. output power as a percent of max rated power. The efficiency is lower when the engine is at lower power and the efficiency rises as the power level increases.
Let's assume that it takes 40 hp to push a Sienna down a level road at 55 mph. If the engine is a 240 hp V6 then the engine will be operating at 16.7% of maximum power (40/240 = 0.167), and will be operating at about 22% efficiency according to the graph. If the engine were a 120hp I4, then the engine would be operating at 33.3% of maximum power and the efficiency would be about 26% according to the graph.
An increase in efficiency from 22% to 26% is an 18% increase in efficiency (4%/22% = 0.18), which is significant.
#9 of 44 Re: 2008 Sienna Hybrid - Do you want 4 cylinders or 6? [jim314]
Jul 06, 2007 (6:05 am)
Very well stated point.
But is 40hp really enough? That number sounds too low for me.
We're not talking about an aerodynamic sedan here, we're talking about a big, boxy two-ton minivan.
I sort of doubt 40hp could sustain speed for a 4300 lb box. And remember, we haven't added the weight for the batteries and the hybrid drivetrain, so with payload we're talking possibly 3 tons to haul.
The 2.4l I-4 in the Camry hybrid makes less power than the regular 2.4l, and just 137 ft-lbs of torque at 4000rpm. So at lower rpm we're talking perhaps under 100 ft-lbs of torque.
You can get some battery assist, but not sustained assist, just an occasional boost.
Can that much torque pull 6000 lbs? Even if it was rated to tow at all, that's what a full hybrid van would weigh.
#10 of 44 Re: 2008 Sienna Hybrid - Do you want 4 cylinders or 6? [ateixeira]
Jul 06, 2007 (10:27 am)
There is undoubtedly accurate data widely available on the exact power (hp or kw) required to push various vehicles down a level road at various speeds, or up grades, but let me show how I think this could be easily calculated from information available inside the passenger compartment.
First the answer: Divide the speed in mph (speedometer) by the instantaneous fuel consumption in mpg (computer readout) and multiply by 12 to get the power in hp that the engine is making at that instant.
A Sienna is travelling on level ground at 55 mph and the fuel use from the computer readout is 27 mpg.
Q. What power in hp is the engine making?
A. (55/27) x 12 = 24 hp
Dividing the speed in mph by the fuel consumption in mpg gives the fuel use in gal/hr. Since each gal of gasoline releases a certain amount of energy on combustion, this quotient is energy per unit time input to the engine. Multiply this by the efficiency of the engine (est 0.25) and you get the power output. We just have to figure out the conversion factor to kw and to hp.
Factor for kw = (1 hr/3600 s)(130 MJ/galUS)(0.25) = 9.0 kw per galUS per hr
Factor for hp = (factor for kw)(1.34 hp/kw) = 12 hp per galUS per hr
The 3600 converts hr to sec. The second factor is the energy content of gasoline in megajoules per US gallon*. The third factor is an estimate of the efficiency of the engine.
A fully loaded Sienna towing a pop-up camper is climbing a serious grade in Galcier NP at 50 mph. The computer readout of fuel use reads 8 mpg.
Q. What power is the engine generating?
A1. (50/8) x 12 = 75 hp.
A2. (50/8) x 9 = 56 kw.
#11 of 44 Re: 2008 Sienna Hybrid - Do you want 4 cylinders or 6? [jim314]
Jul 06, 2007 (11:19 am)
Interesting formula. I guess I'll have to keep an eye on the trip computer to see how far the mileage dips when I'm going up a hill. Don't think I've seen lower than 8mpg before, though.
#12 of 44 Let's put it on layman's terms
Jul 07, 2007 (8:51 pm)
A 4 cyl Sienna hybrid is like having a Camry hybrid powertrain on a sienna body. That may be acceptable in other countries but I don't think it has adequate power for American taste.
A 6 cyl hybrid is like having the powertrain of the new Highlander hybrid on a sienna body. Maybe about 200 lbs heavier than the highlander but more aerodynamic. This will more or less provide similar mileage as the highlander hybrid.
I have a 2006 highlander hybrid AWD (old rating of 27 MPG Hwy and 32 MPG city). I drive 95% highway and I get 31 MPG on hand calculation.
Jul 10, 2007 (10:16 am)
The other thing is that in practice, out in the real world, people with the new V6 are getting very close to what 2.4l owners are getting in similar vehicles.
Consumer Reports measured only a 1 mpg difference between a 4 banger and a V6 RAV4, yet the V6 performed much better. Is it worth a small (5%) gain in efficiency for a big sacfrifice in power and perhaps payload/towing capacities?
Basically, I think the 2GR 3.5l V6 engine is more evolved and efficient than Toyota's 2.4l for its given size.
Toyota should put the engineers that tuned the 2GR in charge of updating their 2.4l 4 banger.
#14 of 44 Re: 2008 Sienna Hybrid - Do you want 4 cylinders or 6? [timothyh]
Jul 11, 2007 (3:57 am)
My opinion, and its only an opinion, is that in order to sell a new car sucessfully (without a $4,000 rebate), the vehicle has to offer something special that competitors don't. It has to differentiate itself.
My opinion is that a buyer who is interested in a hybrid has a greater degree of concern over fuel economy/the environment/conservation than an average buyer. The comment earlier was made that Americans like powerful cars. On average, that's correct. But the hybrid is never going to be successful attracting that average consumer. A hybrid vehicle shouldn't try to compete on power, towing, or acceleration. It's not going to be best in category, and emphasizing those attributes take away from maximizing fuel economy, an attribute where the hybrid excels.
Toyota's goal, probably, is to gain 3%, 4%, 5% (pick any number) market share in the minivan segment by having best in class fuel economy in that segment by a wide margin. I hope that they recognize that most hybrid buyers are willing to give up some acceleration speed in exchange for maximizing fuel economy.
One of the posters noted that Toyota's 6 cyl gasoline engines get about the same mileage as their 4 cyl. That's a Toyota failing that hopefully they will fix.
I'm interested in your opinion as to where my thoughts might be wrong.