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Over-Revving When Accelerating.


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When I'm accelerating passing another car my automatic box seems to be slipping, just like the feeling of a slipping clutch on a manual gearbox. Any ideas, anyone? It seems to over-rev before picking up speed. Is this normal? I've only had the car about 5 weeks and I'm not absolutely 'au fait' with how it should feel. Thanks Chris.

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When I'm accelerating passing another car my automatic box seems to be slipping, just like the feeling of a slipping clutch on a manual gearbox. Any ideas, anyone? It seems to over-rev before picking up speed. Is this normal? I've only had the car about 5 weeks and I'm not absolutely 'au fait' with how it should feel. Thanks Chris.

The experience you describe is "normal" on a 400h, due to the use of a CVT (constant velocity transmission), rather than a regular torque converter type gearbox.

I'll try to keep this simple!

Instead of "gears" there are two pulleys through which runs a steel belt. Each pulley can expand or shrink it's diameter. So at low speed the engine pulley is small & the drive one is large to give the engine (or front electric motor) the appropriate "gearing" to move the car. At higher speeds the ratio is reversed, the engine pulley is large, the drive large, to generate the effect of a high gear. (A look at the way a bicycle with derailier gears will make this easier to understand).

In effect you have one infinately variable gear.

When you accelerate the load causes the ratio to shift back towards a low speed setup (but not that far) to allow the engine to maximise it's power delivery. Although it sounds like the engine is over revving or (as you descibe) the clutch is slipping what is actually happening is the engine is deivering the power you demand as a constant & the transmission is seemlessly adjusting to match that to the road speed as you accelerate.

Yes, it does sound odd, but it is very affective because with not steps in the power delivery to change gear you have constant smooth acceleration, which only relents when you ease off the power.

It requires a different technique to get the best from & practice is the only way I'm afraid! Much like gaining great mpg figures from the Hybrid drive chain as a whole, the more you give, rather than demand, the higher the returns.

This explantion is, I hope, sufficient to make things a little clearer! It also highlights two more small items.

1. Why there is a power meter instead of a rev counter. The latter would be pointless, over revving is impossble because the CVT & management electronics prevent this, the maximum rev points for power & torque are unenccessary because there isn't any gears to require them as gear change points & in this setup knowing how much power you are pushing through the transmission is of more value.

2. The B setting on the gear shift. Because in a Hybrid the engine shuts down in over run mode you don't have it available as engine braking when descending hills. If needed this can be restored by selecting B & thereby over ruling the engine's normal shut down mode & allowing it to provide some retardation, although this may not be as much as with a regular gearbox/torque converter.

There you go ........ now where did I put those asprin tablets...... it's amazing how much your brain just does when driving..........

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When I'm accelerating passing another car my automatic box seems to be slipping, just like the feeling of a slipping clutch on a manual gearbox. Any ideas, anyone? It seems to over-rev before picking up speed. Is this normal? I've only had the car about 5 weeks and I'm not absolutely 'au fait' with how it should feel. Thanks Chris.

The experience you describe is "normal" on a 400h, due to the use of a CVT (constant velocity transmission), rather than a regular torque converter type gearbox.

I'll try to keep this simple!

Instead of "gears" there are two pulleys through which runs a steel belt. Each pulley can expand or shrink it's diameter. So at low speed the engine pulley is small & the drive one is large to give the engine (or front electric motor) the appropriate "gearing" to move the car. At higher speeds the ratio is reversed, the engine pulley is large, the drive large, to generate the effect of a high gear. (A look at the way a bicycle with derailier gears will make this easier to understand).

In effect you have one infinately variable gear.

When you accelerate the load causes the ratio to shift back towards a low speed setup (but not that far) to allow the engine to maximise it's power delivery. Although it sounds like the engine is over revving or (as you descibe) the clutch is slipping what is actually happening is the engine is deivering the power you demand as a constant & the transmission is seemlessly adjusting to match that to the road speed as you accelerate.

Yes, it does sound odd, but it is very affective because with not steps in the power delivery to change gear you have constant smooth acceleration, which only relents when you ease off the power.

It requires a different technique to get the best from & practice is the only way I'm afraid! Much like gaining great mpg figures from the Hybrid drive chain as a whole, the more you give, rather than demand, the higher the returns.

This explantion is, I hope, sufficient to make things a little clearer! It also highlights two more small items.

1. Why there is a power meter instead of a rev counter. The latter would be pointless, over revving is impossble because the CVT & management electronics prevent this, the maximum rev points for power & torque are unenccessary because there isn't any gears to require them as gear change points & in this setup knowing how much power you are pushing through the transmission is of more value.

2. The B setting on the gear shift. Because in a Hybrid the engine shuts down in over run mode you don't have it available as engine braking when descending hills. If needed this can be restored by selecting B & thereby over ruling the engine's normal shut down mode & allowing it to provide some retardation, although this may not be as much as with a regular gearbox/torque converter.

There you go ........ now where did I put those asprin tablets...... it's amazing how much your brain just does when driving..........

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Sorry MartyCee but you are only partly correct.

The 400h does use a CVT (constant velocity transmission), rather than a regular torque converter type gearbox.

However, it is a geared system - not belts, pulleys, cones etc as in some other CVT systems.

Here is a link to a site with a clever interactive diagram that describes how the CVT works in the Prius but it is essentially the same as the Lexus.

http://www.eahart.com/prius/psd/

Hope this helps.

JBP

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