A spin to cruise control

[RU482]

When driving, I occasionally get the urge to try an hypermile when there isn't much traffic.

While doing this the other day, I got to thinking about a different form of cruise control. I find it very hard to believe that CC saves gas, because on mild hills, in order to maintain speed, it kicks the transmission down a gear way to often. If I drive "by foot", I can keep the car in top gear and only slow down slightly.

I was thinking it would be cool to create a controller that would maintain an RPM instead of a specific speed. Obviously, there would be a range of MPH (+/- 10MPH, for example) that you would want to maintain.

For example, in my Impala, at 65MPH, I run at about 1500 RPM. So the controller would be set to 1500 RPM, with a MPH range of 55 to 75.

Thoughts???

 

[herm0016]

it may work, but would suck for everyone else on the road. cruse control saves gas because you are not accelerating nearly as much as you would with your foot. it attempts to keep you at a constant V so you do not use gas speeding up, or wast energy slowing down.

 

[RU482]

Originally posted by: herm0016
it may work, but would suck for everyone else on the road. cruse control saves gas because you are not accelerating nearly as much as you would with your foot. it attempts to keep you at a constant V so you do not use gas speeding up, or wast energy slowing down.

I guess I try to "drive smarter than the Cruise Control"

For example, where I live there are mild rolling hills. With the cruise control enabled, the car is constantly engine braking downhills (slightly) and downshifting going up hills. The downshifts bounce the Tach up to 3k+, and send the instant economy down to the single digits.

When I drive by foot, I typically drive around 65MPH. Going up hills, it can get as low as 57 MPH. Going down hills, it can go up to 75MPH. I try to maintain a constant RPM to maintain the instant fuel economy (and eventually, the average fuel economy). Very rarely does the car need to downshift. I take full advantage of the downhill momentum as well.

This is the experience behind my idea.

I'm in Iowa, so it's not like there's a ton of traffic. And when there is, I drive with the flow.

 

[bobsmith1492]

In a manual the RPM is directly correlated to the wheel speed with a ratio determined by what gear you are in - hence it would be exactly the same.

Automatics are more complicated; the transmission downshifts when you give the engine enough gas that it can rev up high enough to drive in the next lower gear. Then there's the torque converter which I don't fully understand.

However, if you were to set the RPM to a fixed level in an automatic, I can guarantee that odd things would happen and you would not appreciate the results. I think you'd get stuck in whichever gear you were in but when you hit a hill ... your speed (hence RPMs) would decrease, so the cruise control would give more gas to the engine. Once enough gas was being sent, the transmission would downshift to make use of the available power. The RPMs would go up so the cruise control would send less gas - the transmission would upshift and the RPMs would go down. Basically it would get stuck in an unstable control loop since (I don't think) the cruise control has any control over an automatic transmission.

 

[CycloWizard]

This would be very easy to do - much easier than maintaining the velocity. However, holding constant RPMs would affect the speed of each car differently. This would cause massive homicide on the roads. That's bad.

 

[firewolfsm]

If you're paying attention and aren't an idiot, driving yourself should use less gas. However, on long drives (2+ hours) you will lose concentration and change your speed without realizing it, then waste gas getting back to what you want.

 

[JSSheridan]

The control systems used by my CC seems fairly rough, on moderate hills my car is 5 mph slower than I want it to be. I drive a manual. Does anyone know what order the control system is typically? While we're on it, does the car's electronic control unit determine how much gas to inject into the cylinder by reading the engine revs or the air flow sensor?

 

[CycloWizard]

Originally posted by: JSSheridan
The control systems used by my CC seems fairly rough, on moderate hills my car is 5 mph slower than I want it to be. I drive a manual. Does anyone know what order the control system is typically? While we're on it, does the car's electronic control unit determine how much gas to inject into the cylinder by reading the engine revs or the air flow sensor?

It depends completely on the car. My old car's cruise directly controlled the throttle. My new one is fully computerized and doesn't even have a physical throttle cable. The old one relied on a simple feedback loop using the difference from the setpoint velocity as the error. The new one may use other indicators as you mentioned, but it's too complicated now for me to look at it and tell.

 

[heymrdj]

You'll burn more fuel making the engine grunt at 1500RPM wide open (wide open for that range, aka the governor dumping gratuitous amounts of fuel in), not to mention running that way is extremely bad on a motors maintenance. Take an engine that's ran at 2,200RPM under light load and 1,500RPM under heavy governor-wide-open for the same miles and open them. The 1,500RPM will be full of deposits and sludge.

 

[RU482]

Originally posted by: heymrdj
You'll burn more fuel making the engine grunt at 1500RPM wide open (wide open for that range, aka the governor dumping gratuitous amounts of fuel in), not to mention running that way is extremely bad on a motors maintenance. Take an engine that's ran at 2,200RPM under light load and 1,500RPM under heavy governor-wide-open for the same miles and open them. The 1,500RPM will be full of deposits and sludge.

If that is true, why does my car stay in the 20's MPG wise if I drive by foot and keep it in top gear, but if I let it downshift, it tanks to the low teens (according to the computer instant mileage)

 

[Rubycon]

Originally posted by: redly1

If that is true, why does my car stay in the 20's MPG wise if I drive by foot and keep it in top gear, but if I let it downshift, it tanks to the low teens (according to the computer instant mileage)

Is it just looking at MAP or a combination of things?

CC based on crank RPM would not work outside of a CVT. Why don't all cars use CVT now? The ECU can keep the RPM in the peak of the powerband majority of the time and it's so much smoother too.

 

[herm0016]

Originally posted by: Rubycon

Originally posted by: redly1

If that is true, why does my car stay in the 20's MPG wise if I drive by foot and keep it in top gear, but if I let it downshift, it tanks to the low teens (according to the computer instant mileage)

Is it just looking at MAP or a combination of things?

CC based on crank RPM would not work outside of a CVT. Why don't all cars use CVT now? The ECU can keep the RPM in the peak of the powerband majority of the time and it's so much smoother too.

because people are stupid and don't like the "feel" of not shifting gears, and also they will not handle the torque of larger engine. still should be in more smaller cars though.

 

[XR250rdr]

Originally posted by: JSSheridan
The control systems used by my CC seems fairly rough, on moderate hills my car is 5 mph slower than I want it to be. I drive a manual. Does anyone know what order the control system is typically? While we're on it, does the car's electronic control unit determine how much gas to inject into the cylinder by reading the engine revs or the air flow sensor?

My mom's Tacoma has a MT w/ CC. Climbing hills it will easily drop 5+ MPH below the set point because the truck is already at WOT and can't go any faster in 5th. An automatic would downshift and remain closer to the set point at a smaller throttle opening. In those cases I will override CC, downshift then resume CC when the road flattens out.

A modern fuel injection system will use a number of variables in determining fuel injector pulse width including, but not limited to engine speed, throttle position, MAP and/or MAF, coolant temperature, intake air temperature and O2 sensor readings. Some cars have more inputs than the ones I listed.

At WOT its often open loop operation. Most cars have a static map that will maintain a slightly rich mixture over most of the powerband. Narrow band O2 sensors are useless for reading AFR once it deviates from stoichiometric (14.7:1) so the system reads MAF/MAP input and adds the amount of fuel it thinks is correct.