# Shifting Efficiently

How do you calculate where the most efficient shift point is? I guess you could apply this to the robots, but my question is actually for automotive transmissions. I can drive a stick shift fine, and have for a few years, but I’ve had no instruction other then a friend talking me through it and giving me about an hour total of ‘shift about now’.

I’m not even sure what you need to be able to go about figuring this out, but with gas prices what they are, any gained efficiency is more money in my pocket.

Wetzel

in mpg efficiency, shifting into a higher gear close to stall. which means running the engine with the least rpms possible. not too good for the engine in the long run. but it doesnt use as much gas. ( idont think… correct me if im wrong)
and for acceleration, read http://www.allpar.com/eek/hp-vs-torque.html which comprehensibly teaches why shifting at redline would be the fastest way to accelerate

I may be wrong but running close to stall means that the engine has to work harder to maintain speed thus using more gas. Every vehicle is different. A lot depends on gearing. I get my best gas mileage at about 1800 - 1900 prm.

As already stated, it depends on the car, more so on the engine in the car. A good visual is a chart showing the power and torque curves like this one . The darker line is the torque curve and the lighter line is the power curve. As you can see, the torque peaks at just over 4000 rpm. With my understanding, the most efficient time is to shift right at the top of the torque curve, however, if you do not need to gain much more speed, shifting at a lower rpm is probably more efficient. Also, the power efficiency is such that shifting at the top of the torque curve should give you the best acceleration, not shifting at redline as greencactus said. Running the engine at lower rpm’s is better in the long run. It is more efficeint on gas and causes less wear on the engine. An engine will last the longest if it is running smoothly at the lowest possible speed without stalling.

Ok… This is an electrical guy talking so I apologize, in advance, and invite the MechE’s to correct me.

For DC motors, max torque is at stall and max speed is at no torque. Maximum power is at 1/2 speed and 1/2 torque. Each gear ratio will have a power curve associated with it. You want to shift exactly where the power curves for each gear ratio intersect.

I’m not sure if this applies to the ICU but I believe it applies to our robots.

Is there at Mechanical Engineer out there to set me straight?

Mike,
You are 100% correct as far as robots go. I’m not familiar with ICU’s but that is how DC motors work.

Refer to attachment:
This is just a quick graph showing some random motor (a chiaphua, actually) with 4 different gear ratios. 1:2, 1:1, 2:1, 4:1.

You can see in the plot how in order to maintain optimal power usage at all speeds, you need to “bounce along the peaks”, and shift where the plots intersect. Ideally, you could use a CVT to vary your gear ration and ALWAYS stay at max power (rather than bounce up and down).

Hope this makes SOME sense,
John

Well any internal combustion engine has more complicated & interdependent processes going on than a DC electric motor, so simple absolute linear rules of behavior are not there (peak torque at half max rpm etc).
The processes such as combustion, gas thermodynamics, compressible fluid flow of the air in/exhaust gasses out are all interrelated and vary with engine load, rpm, air temperature, resonant frequencies of the intake/exhaust plumbing etc.
However, that being said keep in mind that all ICE are heat engines which function by converting the heat energy of a fuel into expanding a gas that pushes on and moves the pistons/crankshaft.
So work is done - force x distance.

Here are rules of thumb I keep in mind:

1. I recall from somewhere that the maximum combustion efficiency of a motor is at it’s peak torque (maximum area under the torque/speed curve).
For most car motors that’s around 3000 to 4000 rpm.

2. On the other hand, internal sliding friction of the pistons/bearings is essentially linear for each revolution of the motor. So, all else being equal, if you can get your desired output hp (say to maintain 30 mph) at half the motor rpm, you will eliminate half the engine revolutions and half the sliding friction in the motor to go a given distance. Less wasted energy in the motor = higher efficiency & mpg.

3. The optimum efficiency for a given motor is at lowest rpm that still gives good combustion efficiency. Crossover of the friction loss & combustion efficiency curves. Motor’s with good torque at low speed, good combustion effieciency and low thermal losses win.

…Any FIRST engineers out there from the motor city with some more detail? Hey speak out FORD, GM, Daimler guys!

I checked with a knowledgeable person on this subject and I will try and sum up what I think I heard. Jess is right on the subjects so far. My friend also clued me into modern drivetrain design and efficiencies and suggested that the lower the engine speed and the higher the gear tend to be the most efficient but are vastly affected by the control computers cutting in to modify operation (i.e. trans and engine). For the most part the design shift point takes into account all of the variables mentioned above with the addition of the designers choice for reserve power, i.e. being able to speed up without having to downshift. ( I am going to make a vast departure here and make an assumption.) I am guessing that in high gear at say 60-70 MPH is likely the engine RPM at it’s most efficient point. For most cars I have driven (other than a Mazda RX 4) that seems to be in the 3000-4000 range the same as Jess pointed out above. My Honda and Toyota four bangers in the 80’s shifted lower than that (about 2500-3000) but didn’t have a lot in the way of engine mod and emmission control.
Now my dad ( a mechanic) always taught me to feel and listen to the engine, it’s trying to talk to you. If you shift too soon, the engine balks (a lot of power vibration) until you speed up (more RPM). Shift too late and the engine is screaming at you to shift. At the right point, the engine is happy, the trans is happy and you are happy.

On my RX 4, 4000 RPM=70 MPH, red line at 7000, 10 MPH/500RPM you do the math. I never found a point where power fell off. Oh WOW!

Thanks Al. I’ve always driven by listening to the car, but didn’t realize it until I was driving on a nice day, but the windows down and music up, and started having some problems in heavy traffic. Music down, shift happy. :rolleyes:

I guess I’ll be content with a happy car. I think I’ve matured past having my engine scream at me.

Wetzel