Seems Like Tranny is Slipping

keep the rpms up.. letting off just tells the car you want to go slower. so.. keep it floored if need be. there is no slippage.. there's only electrical draw. you have to produce enough electricity to keep the Hybrid Synergy Drive working...

the Internal Combustion Engine is limited... so i won't tear itself apart ( no matter what it sounds like )

keep driving.. have fun.. and keep the speed up. it's embarrassing when i have to pass you guys on the grade.

 

Hi M8's,

Nothing is wrong with your car. It has a small engine. The HSD transmission has a large ratio range. So, when you press hard on the accelerator this system allows the engine to go to full power, which for all engines is near their maximum RPM. The difference is that the HSD transmission has a ratio range which allows the engine to go to full power even when the car is at lower speeds. Unlike the traditional automatic transmission and torque converter.

Usually its not the torque converter that slips in a worn out automatic transmission, its the mechanical clutches (sometimes called "bands" ie band clutches). When a traditional automatic transmission slips, the power does not make it through the transmission, it just makes the metal hot, and burns the lubricating fluid. That is not happening in the Prius.

This combination of wide ratio range transmission and a small engine makes for the best fuel economy. As the partial power fraction of the small engine is larger, than if you have a large engine, when your running the car in the average crusing. And so in cruising the power the engine is run at is is much closer to the optimum efficiency power and RPM. As the throttle plate can be operated closer to full open , and that reduces pumping losses. So, what the car wants to do is get that throttle plate as far open as it can, for the power required.

Now if the battery is low, the car will act to keep it from getting too low, as its an expensive piece of the car. And if allowed to discharge too low, the battery can be damaged and that would cost Toyota some money. If a low state of charge happens on a steep portion of a mountain road, the computers will not allow the battery to assist the road power anylonger. So not only is the car not not getting the usual battery assist, but instead its not getting all the engine power either, because some has to be diverted to recharge the battey. Remember, the car does not need to be going fast, for the engine to rev full speed. The extra power not going to your wheels, in this situation, its going to the battery.

To avoid this, as you start up a hill, look at the MFD, and do not go beyond a speed where the yellow battery assist arrow comes on, continuously. That is, run at a speed where the engine is supplying the power only. Momentary increases in slope are OK for battery assist, but if there is not a lower slope area where the engine has enough power to push the car at the speed you want, and still recharge the battery to the 60 % (highest blue bar) state of charge it wants to maintain the battery at, you will eventually cause the computers to take engine power away from the wheels, and send it to the battery. And you will slow down.

To sum this up, the average power of the car is no more than the 76 hp of the engine. The peak power can be 110 HP, and the lack of shifting and wide ratio range of the engine allows the car to accelerate similar to a 140 hp car with a traditional 4 speed automatic transmission. This is not a big problem in most situations. Since cruising on level terrain at 70 mph requires only about 30 hp. So, if you deplete the battery in a rapid accelleration up to 70 mph, even up hill, once your cruising at 70 on level terrain the engine will quickly recharge the battery. Moutain climbing is another matter. And you need to drive at a speed that does not use the battery, on average, untill your near the top, and know you wont deplete the battery all the way.

 

The basic idea isn't getting through to you, so let me give it a try:

1) The Prius does not have an automatic transmission in the normal sense. There are no brake bands, torque converters, or clutches. It is impossible for the transmission to slip, unless you actually grind off the gears.

2) You cannot over-rev the engine in a Prius. Engine speed is controlled by the computer, not by your foot. It is limited to about 5,000 RPM, which is low by today's standards. The engine sounds loud at higher RPMs because of the engine type and the lack of sound absorbing material under the hood.

3) On a long uphill climb, there is a good chance that the battery SOC will draw down to the low limit. At this point all of the power is coming from the 1.5 liter ICE.

What does all of this mean? It means that the ICE has to work like crazy when you go up a long hill, but that's okay because it was designed for it. You can't overwork the engine, so if you need to go faster, press harder.

Tom

 

Yes, that's it exactly. Anyone used to a "normal" car has become accustomed to a direct relation between car speed and engine speed. Granted, the gearing will change this ratio, but only by steps. With a regular car, when you step on the gas, the engine picks up speed only as the car picks up speed.

The Prius is different. The transmission on the Prius does not directly couple the engine speed to the speed of the car. The car can move without the engine, and vice versa. It can be disconcerting at the beginning, but once you get used to it regular cars seem weird.

In the situation you describe, here is what happened:

1) You are cruising up a long, steep hill. The battery SOC goes very low from providing electric assist. Once the battery hits the low limit, all power is supplied by the ICE.

2) Without electric assist, the ICE must work harder to keep the Prius going up the hill. The control system will increase the ICE RPMs if necessary to provide this power in an optimal way. The car won't go any faster, but the engine will spin faster to make up for the lack of battery power.

3) You step on the gas, telling the control system to go faster. This will require more power from the ICE, most likely resulting in more RPMs. Depending on the steepness of the hill, the ICE may not have much power left in reserve. When the ICE goes to wide open throttle, the ICE makes a lot more noise for a little more power. If you stay with it, the car will slowly accelerate and you will gain speed, but it may take awhile.

4) With the low SOC, the control system will attempt to recharge the battery as soon as possible. This will cause the ICE to run harder as it makes energy for both propelling the car and charging the battery.

Tom

 

I have driven several times from Phoenix to Flagstaff in several different (rental) Prius. Never had a problem keeping up with traffic or with the battery SOC becoming low. The engine does get loud though and instantaneous MPG drops into the 17MPG range. At first I felt uncomfortable with the engine revving the way it did, but I soon realised that no damage was being done. Driving back the other way the mileage was great.

 

Short answer: don't expect to be able to pass cars on long climbs.

 

Okay, several good responses. I'm in the Phoenix area, travel frequently up the large grades to Northern and Eastern AZ. The Prius can be a very quiet car or a very loud car. On a flat road the engine is actually running slower than a traditional car, and on a mountain road it's spinning faster. I've been told my car is quiet, and I've caught people off guard on mountain roads with how the engine can race without a direct relation to acceleration. I will tell you that I can keep 85-90+ MPH with no problem with 525lbs worth of people in the car going up the long Phoenix grades in the dead of summer with the A/C on. I can slow down behind a slow truck, and get right back up to speed. It just takes a bit RPM. My other vehicle has 425hp/425tq so I'm used to power going up hill. Although it doesn't put you back in your seat, the Prius builds power very linearly going up hill and keeps pulling. I've never been disappointed at how fast I can go up a hill in my Prius loaded... once I got over the initial sound of everything hitting redline all at once.

I'll give my spin, but it's basically just adding to what was already said. There's basically 3 forces at work here. There's a ICE, there's a MG1 motor, and a MG2 motor (remember, a motor is also a generator when spinning). The MG1 motor is a smaller motor, which is the one that also starts the ICE. The MG2 motor is the larger one. The MG2 spins in a direct proportion to the drive wheels. For example, if you double your speed from 10mph to 20mph the MG2 will also double it's speed. That's the only constant here. The MG1 and the ICE work together to keep the MG2 going as fast as it wants to go (which is dictated by what your right foot does).

Let me preface the following as an attempt to offer a new spin on how things work. I'm being very simplistic here.

Engine RPM in a vehicle with a CVT (although a Prius isn't really a CVT, the motors and engine working in conjunction makes it feel the same) does not relate directly to vehicle speed (exception would be the MG2, but you don't feel that ). The Prius' computer determines how much motor and engine to use to get the optimal economy based on demand. In a way you can think of a Prius as a axle differential with an electric motor on one side of where a wheel would go, and a engine on the other.

Hopefully this pic shows up. Imagine having an engine on one side, and a motor on the other. In the center would be where the power is outputted (this is how I'm illustrating my example, obviously in real life a differential works just the opposite.)



Where the yoke would be, that's what's driving the wheels (this could be represented by the MG2 motor as it's speed is directly relative to wheel speed). Anyway, imagine you take your hand and spin the left side while holding the right side. The center will spin in relation to the left side. Now spin the left side and spin the right side. You're now transmitting the power from both sides in to the center. The left and right side can spin at different speeds and can even cancel each other out completely if you spin them opposite to each other at the same speed (without the yoke moving). This is similar to how the Prius' HSD allows the motors/engine to operate at different speeds relative to the actual speed of the vehicle.

Let's say that "yoke" requires 3,000 RPM to go a certain speed (assuming a 1:1 ratio for simplicity). The computer determines that based on low load that it can get that from the electric motor alone. The motor spins at 3,000, the engine is at 0, and that fictional yoke spins at 3,000. Let's now add a gradual hill to the mix. The yoke still needs 3,000RPM but the increased load requires additional HP. The engine comes on line and revs up to 5,000 RPM. Since the yoke still wants 3,000 RPM, we have an interesting situation. The engine is actually turning faster than the yoke (wheels) need to go. Oh no, you're going to speed out of control and die because the tranny doesn't really have gears. In this situation the computer will actually not only cut the motor, but actually run it backwards. You're now running with the engine forward at 5,000 RPM and the electric motor (mg1) backwards at 2,000 RPM to make that 3,000 RPM the wheels need. How much engine/motor is being used is determined by things such as SOC, motor temp, load, etc.

 

Nice explanation, but a correction.

The engine is always spinning if it's running; there is no clutch. So the electric motor turns backwards if you need no output power (like at a traffic light while the engine is warming up.)

 

In engineering, there are often many ways to approach a problem, each equally valid yet on the face of it, very different. An energy analysis is another such approach. This is just another way of looking at how the world works.

Energy comes in different forms but we often deal with:

• drag energy - coefficent*cross_section*velocity*velocity*velocity
• kinetic energy - (1/2)*mass*velocity*velocity

What this means is the energy needed to overcome drag increases by the cube of the velocity while the kinetic energy of the vehicle increases by the square of the velocity. But the maximum energy available from our engine remains constant, fixed at the maximum engine rpm, 4,500 rpm in my older NHW11 and 5,000 rpm in the newer NHW20. But as the velocity increases, the car accelerates slower and slower:

Notice how as the speed increases, the car takes longer and longer to go faster. The engine power is coming in balance with the vehicle drag and less and less power is available to go faster. To do this maximum acceleration test, the engine was pegged at 4,500, the accelerator flat on the floor. Your NHW20 does this a little faster yet the overall shape of curve is similar.

So let's take a look at the power needed as a function of speed:

Notice how higher speeds soon reach a point where any incremental increase takes a lot more energy.

When going up hill, the reserve engine power is soon absorbed by:

• potential energy - gravitational_constant*mass*(height_1-height_2)
• potential energy ~ gravitational_constant*mass*velocity*sin(slope)

What has been missing is the grade. What I've found is a 6% grade at a speed of 55 mph provides an optimum climb speed for sustained climbs. My NHW11 can go up at 60 mph but anything over 60 mph on a 6% grade runs into an energy deficit. In these cases, the battery provides the extra energy and depending upon the size and grade of the hill, the battery can run out.

Bob Wilson

 

Whoops.... I saw the title of this thread and completely got the wrong idea. Um, never mind. Carry on.

 

+1 recognizing the many posters and different perspectives offered in this
thread. Kudos to all you all.

It has been an education for me too. Many thanks.