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Could cars have a better fuel economy at low speeds?

  1. Sep 26, 2015 #1
    The part that I roughly understand:

    1) Drag equation, drag increases by square of velocity, thus if it was only factor to be taken in to account then the lower speed the better.
    2) In case of cargo ships there is a policy of decreasing speed to reduce fuel usage, called "slow steaming". It became really trendy during recent crisis as there was excess transport capacity and very price conscious clients.
    3) Actually for cars the fuel efficiency picture shows also problem of too low speed:

    Fuel_economy_vs_speed_1997.png

    OK, so my question:
    -why low speed reduce so drastically fuel economy of contemporary designs?
    -if one wanted to design a car that works perfectly fine (or even better) with low speed, how should be the engine (?) redesigned?
     
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  3. Sep 26, 2015 #2

    DaveC426913

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    To take that logic even farther, you can get infinite efficiency at zero speed.

    The primary purpose of a vehicle is to get from A to B, as quick as safely reasonable. Fuel efficiency is only secondary to that.

    Change the gear ratio so that the engine is running at optimal RPM for whatever speed it is doing.
     
  4. Sep 26, 2015 #3

    SteamKing

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    This is true, if that's all the constraints which you must satisfy. But cars are typically designed to operate most efficiently at highway cruising speeds. There are, however, some so called "city cars", which are essentially mopeds with two seats side by side enclosed in a small cabin. These cars have tiny moped engines and are not designed to exceed 45 kph or so.
    Cars and ships operate in very different environments. While marine fuel may not be highly refined like gasoline or diesel fuel, its cost is not inconsequential, and even small ships burn tons of the stuff every hour they operate.
    The problem with fuel efficiency at low speeds for cars is not so much the speed, but that in urban areas where cars are obliged to go slow, there is a lot of stop-and-go driving, which tends to drive down the fuel mileage of even the most efficient designs. If your car is idling while stopped at a traffic signal, or while you are stuck in a traffic jam, even the most efficient engine registers 0 liters per km or miles per gallon.

    Alleviating congestion, unfortunately, is one problem which even the most talented engineering can do little to overcome.
     
  5. Sep 26, 2015 #4

    mfb

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    The gear is an important point. The engines have some RPM range where they have their best efficiency (RPM is related to the timing in the cylinders). Friction in the engines doesn't depend so much on what happens outside, so you have some consumption "liters per hour". That ruins the efficiency if you are too slow.
     
  6. Sep 27, 2015 #5
    Assuming that one does not for example operate in area where there is a very strict speed limit. Like for a example a city.

    OK, with extra googling I think I roughly found the technology that I roughly thought of, it's called "Variable displacement". Any ideas to what extend this is practical?
     
  7. Sep 27, 2015 #6
    In order to accelerate or keep a certain speed, you must use fuel of some sort. In order to accelerate faster or maintain a faster speed, it will require more fuel. I'm not sure if anyone else said this as I simply scanned the replies, but this is, in my eyes, a simple way to look at it. I hope this answers your first question.
     
  8. Sep 27, 2015 #7

    SteamKing

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    If you kept Googling, you should have come across this article:

    https://en.wikipedia.org/wiki/Variable_displacement

    As with a lot of automotive technology, it's a case of "Been there, done that."

    While certain engines in current production are fitted with so-called variable displacement engines, it is a technology which has only been fitted to certain makes of engine. There are different methods of cylinder de-activation employed, but they all add complexity to the engine. The biggest gains in fuel economy come from fitting the technology to large, multi-cylinder engines, but in congested urban areas, the current types of cylinder de-activation methods don't give much increase in fuel economy, since cylinders are shut down usually only after cars reach cruising speed, like on a highway, where a high, steady speed desired.

    Since computerized engine controls are now pretty much mandatory to allow manufacturers to meet strict emissions and fuel economy regulations, further development of such concepts will continue. One feature which might eventually find its way into production is replacing the camshaft used to open and close cylinder valves with electronic actuators which would do the same job. Because the valve actuation would be electronic rather than mechanical, the valve timing could be more accurately controlled under different types of driving situations, and cylinders could be switched on and off as need be.
     
  9. Sep 27, 2015 #8

    Nidum

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    Getting good overall efficiency in slow speed and stop go driving conditions is one of the reasons that hybrid car designs with engine plus battery plus electric motor drive are being developed .
     
  10. Sep 27, 2015 #9

    mfb

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    To be faster, you need more fuel per time. That is obvious. The ratio "fuel per distance" is not that obvious, when you are faster you also make more distance per time.
     
  11. Sep 27, 2015 #10

    Vanadium 50

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    You can model this as two pieces to the fuel consumption. The first part is simply proportional to the time the engine is running. If you go slowly, this part will dominate. The second part is a function of how fast you are going. If the second part is linear, the mileage goes as a/v + bv.

    That model works surprisingly well at fitting the data.
     
  12. Sep 28, 2015 #11

    This brings a very challenging question. I have not researched this heavily however I have worked on several hybrid and Composition drive experiments. My initial assumption would be that the lower efficiency peak is closer to the actual peak if one was only dealing with the aerodynamic efficiencies. The upper peak would be closer to peak efficiency of the engine and drive train.

    Engines have a relatively narrow band where they are able to be efficient. The entire rest of the range where they are driven we are forcing them to operate outside of their inherit peak. The most common and relatively effective direction chosen is to allow lower speed aero efficiency and pursue higher mechanical efficiency where the people want to drive. The hardest job you will ever encounter is combatting the "group think" that faster is better. this is a public perception and not a mechanical design issue. The people will often override your good design when it does not suit them.

    I know this is a long way round to answer your questions however, The best efficiencies I have ever been part of were slow speed vehicles. This was coupled with an engine that was tuned to produce peak power at its peak efficiency. The engine was used directly to drive a generation plant for storage. The energy that was stored was used directly to drive the vehicle. Down sides being it was a heavier drive train (possibly overcome by modern technologies), it spooked the drivers ( when the engine starts and stops on it's own and "races" it is just plain weird), and the efficiencies suffer at higher speeds.

    This must be contrasted to the results. In one particular the 4500 lb. test bed vehicle achieved 84 mpg in city driving which was partly due to efficient regenerative braking. The same vehicle only achieved 45mpg on highway at 55 miles per hour.

    Your initial post queried what kind of drive train redesign would be needed. It is my thought that a small displacement engine. One that was highly tuned and as efficient as possible at generation direct to storage( probably electric but hydraulic storage is also possible) supplying a drive train that can run efficiently at slower speeds would provide the best results. I would then add a feature on the dashboard. When the driver started to exceed that best efficiency zone it would warn that peak efficiency was no longer possible. Some or many of modern drivers would adhere the rest already know they are not fuel efficient. This would then provide your goal and their choice.

    I know, this is a lot of engineering and even more crazy junk to provide. but hey you asked what I would try.
     
  13. Sep 30, 2015 #12
    The reason the national speed limit was 55mph for awhile had nothing to do with safety. It was to conserve gas during the OPEC oil embargo. Like someone said before, don't move at all, costs zero energy. There were much less fatalities in the highway, because people stopped using them, studies would later find. In 1982 they lifted it. Gearing does help to a degree but eventually you might as well build an airplane because the drag becomes the major factor. I'm renting a 2015 Dodge Charger right now that easily does 30 mpg on the highway with the cruise control on at 70mph. Not bad for a pretty big heavy car, but that's only to maintain a steady speed. Most newer cars you can visibly see the current mileage you are getting (or buy $25 OBD-II/Bluetooth adapter and there is a torque app $5 that will give you the same read out on any 1996 or newer car). It's fun to play with. Now hybrids can get ridiculous city mileage but that's because of recapturing energy during deceleration. They are typically heavier though and often aren't even as good on the highway as cheaper car. Ultimately though the slower your average speed, but exponentially more important the slower you accelerate the less energy you will use. Of course time is the one thing you can't really get back, money is easier comparatively, so the only time you'll catch me driving on the cruise control is if skipping a fuel stop during the trip saves me those few minutes somewhere else.
     
  14. Sep 30, 2015 #13

    rcgldr

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    A general purpose car needs to be able to travel at highway speeds (one Texas toll road has a speed limit of 85 mph), while going up moderate grades. That requires a power to weight ratio that results in an engine that's inefficient at speeds below 20 mph. Hybrid cars get around this by having two engines, one electric, one gas, a battery to store energy, and regenertative braking. For a hybrid car running at low speed or low power mode, it can run off the electric engine and battery, cycling the gasoline engine on and off to recharge the battery while running the gasoline engine at an efficient power output. The gasoline engine and electric engine can both run at the same time for increased power for a short period of time, with the combined power of the gasoline engine and the battery (at it's maximum discharge rate).

    An alternative to variable displacement would just cut fuel to certain cylinders in a pattern to keep the heat evenly distributed (rather than mess with the valves). This is how traction control is often implemented.
     
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