Short trips & bad mpg?

J

JaCkaL829

it was my first time...
Hey guys,



My title doesn't really indicate a problem, as I already know short trips usually yield low mpg, but I'm just curious as to why? From what I read online people mention it's because the engine isn't warmed up, but never elaborate more. I've just read up more on car engines and the difference between open/closed loop operation, I'm wondering if this plays into as well as the 'warming up' period? Correct me if I'm wrong but most cars run 'rich' while cold similar to a carburetor engine with the choke on all the way when it's cold.



I know two people who get terrible gas mileage, 13-14 mpg from midsize sedans (05 Accord and 08 C300), who have very short commutes less than a mile or 2-3 total minutes, and they're baffled. They also drive very conservatively. I would like to explain to them better why they get poor mileage, but I'm not 100% sure why other then what I mentioned above.



Any explanations on the topic would be much appreciated.



btw before anyone says to service the two cars mentioned above, they both get good mpg on long, highway trips, so it's more their commute then the actual car.
 
I think it's what you already said... rich running while cold. Also, you get much better gas mileage when you take long trips coasting on the freeway with low RPMs.
 
It's not as simple as running rich, but that's part of it. The basic explanation is that you're converting the fuel to heat. The heat expands the combustion products, driving the pistons up and down. A vehicle is not very efficient at capturing this heat; in comparison to say a modern condensing boiler/furnace in your house, which captures so much of the heat for heating your house that it can use a plastic (PVC) exhaust pipe. In your car, a lot of the heat goes out the tailpipe, and a lot of it is absorbed by the various metal parts of your engine, and the coolant and oil. If you do a lot of short trips, you keep using the fuel energy to "re-heat" all those parts of your engine, especially during the winter when they cool down quickly.



The open loop/closed loop different is similar but different. The O2 sensors and the catalytic converter don't operate properly until they reach a certain temperature. Since the engine control computer can't read the O2 sensor it can't be sure the combustion process is tuned properly, and therefore uses a pre-programmed "open loop" fuel map based on throttle position, MAP, etc. As noted, this mixture is rich.



I think the easiest way to visualize this is to think about the heater in your car. How does this work? Hot coolant is run through a heat exchanger, and air is drawn over that heat exchanger (your heater core) to be warmed and then enters the passenger compartment. If you drive for 20 minutes, stop for a cup of coffee for 3 minutes, and get back in the car, your heat will still blow hot. If you get out for 2 hours to have dinner, it will blow cold. You need to put the heat back into the coolant in order to take it out to warm the air. The heat that you put into the coolant is energy from the gas in your tank, so the more times you do it the more energy you use. That's a little oversimplified, but whatever.
 
Setec Astronomy said:
It's not as simple as running rich, but that's part of it. The basic explanation is that you're converting the fuel to heat. The heat expands the combustion products, driving the pistons up and down. A vehicle is not very efficient at capturing this heat; in comparison to say a modern condensing boiler/furnace in your house, which captures so much of the heat for heating your house that it can use a plastic (PVC) exhaust pipe. In your car, a lot of the heat goes out the tailpipe, and a lot of it is absorbed by the various metal parts of your engine, and the coolant and oil. If you do a lot of short trips, you keep using the fuel energy to "re-heat" all those parts of your engine, especially during the winter when they cool down quickly.



The open loop/closed loop different is similar but different. The O2 sensors and the catalytic converter don't operate properly until they reach a certain temperature. Since the engine control computer can't read the O2 sensor it can't be sure the combustion process is tuned properly, and therefore uses a pre-programmed "open loop" fuel map based on throttle position, MAP, etc. As noted, this mixture is rich.



I think the easiest way to visualize this is to think about the heater in your car. How does this work? Hot coolant is run through a heat exchanger, and air is drawn over that heat exchanger (your heater core) to be warmed and then enters the passenger compartment. If you drive for 20 minutes, stop for a cup of coffee for 3 minutes, and get back in the car, your heat will still blow hot. If you get out for 2 hours to have dinner, it will blow cold. You need to put the heat back into the coolant in order to take it out to warm the air. The heat that you put into the coolant is energy from the gas in your tank, so the more times you do it the more energy you use. That's a little oversimplified, but whatever.
Click to expand...



You have the theory down of energy that can't be destroyed but converted.



Open loop the o2 sensors are not used (scan tool will display 450mv). After the ect reaches 160 deg the ecu determines if it should go into closed loop based on engine and ambient temperature parameters. The reason that the engine runs rich is because the air is denser when cold than when warm. The ecu picks this up based on maf, intake air and ambient temperature sensors. As the engine reaches operating temperatures and the o2 sensors are warm enough they begin to monitor o2 readings and the ecu begins to adjust fuel trim values, thereby adjusting injector pw to provide near perfect 14.7:1 ratio. This would lean the mixture from when you started the vehicle and the ecu read baro and used a calculated value opposed to real-time readings.



The quicker an engine reaches operating temperature the better. For every gallon of gasoline used a gallon of water is produced due to condensation that the oil absorbs throughout the crankcase and is blown into the combustion chambers through the pcv system (early cold morning white smoke from tail pipe, then goes away after warm). So the ideal operating temperature of any engine is 210-220 deg at the temperature water begins to boil.



The question in the topic is fuel economy based on a 2-3 minute drive. To begin with how long do they leave the vehicle running at start up? It has been proven that a vehicle should be left to warm up for couple of minutes and then driven lightly. Reason being if the car is left to sit out for 20 min before driving off well thats gas burned from a vehicle sitting there.



It being such a short distance is it stop and go traffic?? If so the transmission works in gear ranges. in first gear most gear ranges are anywhere from 4:1 (6 speeds) to 2.5:1 (4speeds) which means the engine will have to rotate 4 times before the wheels rotate 1 producing 4 times the torque but requiring 4 times the load. Contrary an overdrive gear is anything under 1:1 like a .7:1 which means the engine will only rotate almost 3/4 turn when the driven wheels have already rotated 1 this creates a .7 torque requirement and only producing .7 times the load.



Finally the most important transmission related factor would be the torque converter clutch. A torque converter uses centrifugal force to double torque. It slings fluid starting from the rotor in the center and hits the vanes at an angle to were they shoot out and hit the cover ends creating a spinning motion much like a water reel that uses water pressure to spin the head and water the lawn within a circular radius. The converter is only used when torque is most needed, at lower speeds. After the vehicle has been driven for a while and the transmission has reached overdrive several times the ecu will determine if weather or not to engage the clutch and stop torque multiplication. If it receives an ok it eliminates torque multiplication and increases fuel economy because of the less load it places on the engine on top of the economy increase because of a overdrive gear. This would require the vehicle to be DRIVEN for at least 10-15 min.



The Last and final thing would be are vehicles in tune?? With 1 spark plug or injector fouled or plugged will dwindle fuel economy because of what the ecu attempts to do to adjust it. The vehicle must be in good shape if they expect it to get the recommended mileage.
 
Setec and the guy above me both had great posts! Your both actually right, but I must ask you what car actually runs stoich (14.7)? None, because then you leave no lee way for bad gas and the such. Take the new Evo X for example. People have recorded 9.8:1 AFR's on bone stock X's. Hence why the car gets such crappy gas mileage, but at the same time it keeps the engine safe and detonation free. The only time I've seen a car run 14.7 AFR is actually at idle. This is was on my car and a few other while being tuned using a wideband (lamda) 02 sensor. Once you get on the gas your AFR's should drop. Not to mention there are cars now running ethanol and thats a whole other can of worms that I'm gonna leave closed for now lol.



To the OP they have answered your question, but this maybe a simpler one. Like mentioned the car isn't operating efficiently until it's sensors and catalytic converter and at "operating" temps. Also depending on whether your car is in open/closed loop mode, which depends on driving conditions (long trips compared to short trips) will greatly affect your gas mileage. If you can hook up a vacuum gauge and run it into your cabin (simply tee off your intake manifold) the next time you go on a long trip. Try to keep your speeds at around 65-70 MPH and keep your vacuum at 0. This will be hard due to just having bumps in the road slightly knock your foot around, but I guarantee you will get incredible gas mileage. I personally do this with my Evo and did it with my turbocharged 240sx. In my Evo, which has over 400AWHP, rather large 780cc injectors and a 255lph fuel pump I was able to do a 300+ mile trip and still had 1/8th of a tank. With the 240sx which had smaller 380cc injectors and 250 to the wheels I did the same trip and had over a 1/4 left in the tank.
 
Tripperfx3 said:
Setec and the guy above me both had great posts! Your both actually right, but I must ask you what car actually runs stoich (14.7)? None, because then you leave no lee way for bad gas and the such. Take the new Evo X for example. People have recorded 9.8:1 AFR's on bone stock X's. Hence why the car gets such crappy gas mileage, but at the same time it keeps the engine safe and detonation free. The only time I've seen a car run 14.7 AFR is actually at idle. This is was on my car and a few other while being tuned using a wideband (lamda) 02 sensor. Once you get on the gas your AFR's should drop. Not to mention there are cars now running ethanol and thats a whole other can of worms that I'm gonna leave closed for now lol.



To the OP they have answered your question, but this maybe a simpler one. Like mentioned the car isn't operating efficiently until it's sensors and catalytic converter and at "operating" temps. Also depending on whether your car is in open/closed loop mode, which depends on driving conditions (long trips compared to short trips) will greatly affect your gas mileage. If you can hook up a vacuum gauge and run it into your cabin (simply tee off your intake manifold) the next time you go on a long trip. Try to keep your speeds at around 65-70 MPH and keep your vacuum at 0. This will be hard due to just having bumps in the road slightly knock your foot around, but I guarantee you will get incredible gas mileage. I personally do this with my Evo and did it with my turbocharged 240sx. In my Evo, which has over 400AWHP, rather large 780cc injectors and a 255lph fuel pump I was able to do a 300+ mile trip and still had 1/8th of a tank. With the 240sx which had smaller 380cc injectors and 250 to the wheels I did the same trip and had over a 1/4 left in the tank.
Click to expand...



You said it, the hardest part about answering a post like this is because there are so many variables. I've never tired to use a vacuum gauge during long trips i will be willing to try that, great idea.



But there is one thing bothering me, isn't the evo turbocharged? The AFR recorded by others, were they during acceleration? This could be a greatest of examples as this applies to the very nature of turbo charging. As the engine is accelerated the charged air that comes in, runs through the intercooler right? Wouldn't the air blown in, be denser than the hot intake chamber?? Not to mention, variable valve timing as the actuators retard timing to increase volumetric efficiency as engine speed increases. Wouldn't it require a higher fuel demand? The fuel trim values are most accurate while coasting. Then again I'm beginning to talk about calibrated values under certain driving conditions. Theoretically the mixture should be 1 lambda (14.7:1). Bad gas would create all sorts of problems as you mentioned. Whats worse its only getting crappier and crappier, oh wait i'm sorry its getting "cleaner" or should i say leaner.



The only bad thing about this is that this could go on and on and on....
 
Theres a solution to this bad mpg on 1 mile trips-



Walk a mile.....leave the car behind



Maybe the actual reason cars get terrible mileage on short trips is because automakers program them to run rich for the first couple of miles so people will start walking more. Its some sort of conspiracy! They want Americans to lose weight in these troubled economic times by walking and consolidating their errands to one round trip!



It all makes sence now!
 
Altered-Image said:
Theres a solution to this bad mpg on 1 mile trips-



Walk a mile.....leave the car behind



Maybe the actual reason cars get terrible mileage on short trips is because automakers program them to run rich for the first couple of miles so people will start walking more. Its some sort of conspiracy! They want Americans to lose weight in these troubled economic times by walking and consolidating their errands to one round trip!



It all makes sence now!
Click to expand...



Yeah i guess i got alittle carried away. School and continous tech training sunk in. Maybe i should get back to :waxing:
 
Thanks for the replies, those we're some detailed responses:xyxthumbs



As for the questions regarding the two people I mentioned. They usually turn their car on and let it sit for 1-2 minutes, then drive. Both of the cars yielded their 'EPA numbers' when driven on the highway for longer drives, so I was more inclined to say it was their short trip, not the car. The Accord is auto, and is driven easy, the C300 is stick and the driver usually leaves it in a high gear with the rpms low, granted it's not bogging the engine. As for where they drive it's mostly residential, with maybe 1-2 traffic lights/stop signs on their whole ride.



I'm little confused with the cold air in the winter. How exactly does the air 'warm' up in the winter time after driving? Is this where the blowback from the PCV system play a role and recycle warm blowback? Or does the heat from the engine bay itself 'warm' up the air that is being drawn in, or is it both? I know on my father's old carburetor'd corolla their is a heat pipe running from his intake manifold to the front of his air intake duct. I remember someone from an auto store was telling me it's to warm up the air in the winter, I guess the same theory would apply to above?
 
Johnny, open-loop is rich for a number of reasons, one of which is your "air" questions. If you take a glass of water with ice in it outside in the summer, it gets condensation on the outside because it's cold enough to condense the moisture in the air which is in a vapor state in the warmer air. As you probably know, liquid gasoline doesn't burn, it's the vapors that burn (hence the lighter fluid trick in those car polish infomercials). The fuel has to be vaporized to burn in the combustion chamber, and when the car/ambient is cold enough, the fuel will condense on the inside of the intake manifold/etc. before it gets to the chamber. That's the reason that those old carbureted cars heated the intake air, to help the fuel vaporize and stay vaporized until it got to the chamber. This problem was largely mitigated by port fuel injection because the injector is moved essentially to the valve port and is squirting on the back of the intake valve.



An internal combustion engine is a heat engine, it's all about the heat, if you lose the heat out of the system, you lose the efficiency. The heat in the expanding gases drives the pistons, some of that heat is lost to the block, etc. Smokey Yunick used to rant about if only he could build a ceramic engine (so it wouldn't melt) that didn't need to be lubricated (the lubricants have a temp limit) he could keep all the heat in the engine instead of it going out the radiator and tailpipe, and he could get great mileage because he could get all the energy out of the fuel instead of wasting most of it. A car has some ridiculously low efficiency after you factor in all the heat loss and all the gear, bearing, and tire friction.
 
JohnnyDaJackal said:
Thanks for the replies, those we're some detailed responses:xyxthumbs





I'm little confused with the cold air in the winter. How exactly does the air 'warm' up in the winter time after driving? Is this where the blowback from the PCV system play a role and recycle warm blowback? Or does the heat from the engine bay itself 'warm' up the air that is being drawn in, or is it both? I know on my father's old carburetor'd corolla their is a heat pipe running from his intake manifold to the front of his air intake duct. I remember someone from an auto store was telling me it's to warm up the air in the winter, I guess the same theory would apply to above?
Click to expand...



I would have to say the engine bay in general. Heat is radiating everywhere under the hood. As for wanting to warm the air, allow me to shed some light.

Oldsmobile Motorsports at Radison 200 - AutoWorld.Com



Though this is an example of how altitude effects engine performance it is very much the same principal. before the map sensor or baro sensor were introduced as people traveled, the higher the altitude the reached the worse their vehicle ran which required frequent adjustments.



The denser the air, the higher oxegen content due to more molecules covering the same space. so the answer is the colder the air that comes into an engine the better.
 
Altered-Image said:
Theres a solution to this bad mpg on 1 mile trips-



Walk a mile.....leave the car behind



Maybe the actual reason cars get terrible mileage on short trips is because automakers program them to run rich for the first couple of miles so people will start walking more. Its some sort of conspiracy! They want Americans to lose weight in these troubled economic times by walking and consolidating their errands to one round trip!



It all makes sence now!
Click to expand...



You have a point,. I noticed my other car really had big problems when I used it often on short trips.. First the low mpgs, then the next thing I knew was that the alternators were going bad. And then the fuel pump and then the radiator.. Geesh.. What a headache..
 
JohnnyDaJackal said:
.... As for where they drive it's mostly residential, with maybe 1-2 traffic lights/stop signs on their whole ride. ...
Click to expand...

Probably a big chunk of it right there (in addition to all the stuff already mentioned).



1-2 traffic lights/stops doesn’t sound like much, but if the whole trip’s less than a mile with 2-3 total minutes those stops are a huge percentage of the overall trip.



Everybody knows that you get worst mileage when accelerating. To accelerate you must pour energy into the car’s motion. The energy of motion is called kinetic energy. It has to come from somewhere and that somewhere is the fuel. The fuel contains (chemical) potential energy until it is burned and converted to kinetic energy by the engine.



So more stops and starts means more accelerate/decelerate cycles and more fuel burned.



In a standard internal combustion powered car all that kinetic energy is simply thrown away in the form of heat whenever the car has to stop. The car’s brakes convert the kinetic energy directly into heat dissipated into the environment (which is actually a form of kinetic energy too, for the air molecules around the car).



Some electric and hybrid cars use regenerative braking, which converts the car’s kinetic energy back into potential energy (charging batteries) that’s stored for later re-use. A car with regenerative braking can get better mileage in town than on the highway.





pc.
 
and engines PCM operates on a seperate set of parameters when started up and when cold. this is called closed loop.. after the vehicle warms up it goes into open loop, which means the computer uses more sensors and uses the input to get u as close to ur stochiemetric air fuel mix (14.7:1 which would in a perfect world, give you perfect combustion)... but thats beside the point.. when your engine is cold, and you are in closed loop, you are running rich, as others have stated.. your idle is higher.. almost doubles in some cases. which it controlled by your IAC (idle air control... think it as the choke on a carbuetaror, kinda..) which allows your pcm to control how much air gets in when your throttle plate is closed ... .... when not at idle, in closed loop you run rich, your engine wants to get up to operating temperatures quickly so it can get youinto open loop and closer to your ideal air fuel mixture, and thats how it does it...engines are not build or designed to be as efficient when cold, theyre designed to operate effieicntly at operating temperature, so your engine wants to get there as quick as possible... im starting to prattle, so ill stop. ha hope it helps
 
My many years of experience suggests that short trips does equate less miles per gallon. However, with my newer cars I purchased in the last five years, the M45, G35, and 335i, the economy difference between short trips and long trips is virtually less than 5%. My result is by no means scientific, but I keep track of my fuel economy every time I fill up, and I do long highway trips two times a week, while the remainder of the week I just do grocery shopping.
 
short version:

fuel delivery is controlled in 2 ways:

by a MAP, the ECU provides preset amounts based on throttle/load/etc, when cold or full throttle

by the O2 sensor fine adjusting the MAP when the engine is warm



when cold or full throttle more fuel is needed, to keep it running stable or generate power without going lean...



when warm and cruising the O2 sensor adjusts the air/fuel ratio (MAP) for maximum efficiency...



in my experience it's significant:

my e46 M3 averaged 24 when commuting 40 miles on the HW

commuting 5 miles HW yeilded 18...>30% delta



my RS4

HW 22

short HW commute 17 or so



my 951

HW 24

short HW <20

the turbo improves efficiency and the control system is much older so less of an influence overall...
 
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