why old car use more fuel to run than the new one?

what i mean that the old car have to use up more fuel to run than the new buy one. i hope to have a right answer,thank you!

2 Answers

  • 4 years ago
    Favorite Answer

    One of the classic inefficiencies of cars of yore is the driveline design. Take for instance a 1962 V-8 Impala with a hydramatic transmission and rear wheel drive. The two-speed transmission always allowed slippage between the engine and the wheels, which caused energy loss in the transmission fluid being heated up in the torque converter during all driving situations. They have cured that in today's automobiles by locking out the torque converter with an automatic clutch. Putting a 90 degree angle between the input and output is also an energy sucker. In more efficient front wheel drive cars, the axis of engine revolution is the same as the wheels. This saves some energy as well. The final drive ratios of cars in the 60's was insane! A cruising RPM of 3600 at highway speeds was not unheard of--and on a V8!!! Think of the gas going down the tubes..... Modern cars reduce the final drive ratio and add overdrive to the transmission to reduce cruising speed RPM. Less RPM means less air consumption and less fuel use. In the 70's, they caught on and offered rear wheel drive cars with 2.73:1 rear end ratios to save gas. This hurt acceleration, but was cheaper than offering a 4 speed automatic. (It eventually happened.)

    A huge determinant of efficiency is COMPRESSION RATIO. Without computer control and injection, most cars of the 70's (BAD efficiency) had LOW compression ratios, say 8.0. (And food for thought--the first spark ignition engines had a sorry compression ratio of 4.0:1. How sad!! They did not know enough about fuel chemistry at the turn of the century, so that's why the horsepower value of vehicles was so low back in the teens. A four cylinder automotive engine having 10hp in the "old days" with a displacement of 2 liters. WOW!) New cars have all kinds of tricks to increase compression ratios while running on regular 87 octane fuel. Current compression ratios include 9.0 to 10.1. Computers adjust injection and spark ignition timing (to repeat a collegue's information already submitted) along with monitoring piezoelectrick "knock" sensors which make adjustments for preignition "pinging." In the old carbureted cars, even ones with HEI ignition, there was no way to deal with the pinging. The sure fire way to prevent pinging (and serious damage) is to lower the compression ratio.

    Another huge problem with cars of the 70's and 80's were the addition of the catalytic converter and other emission control systems like the huge drain from a "smog pump." EGR valves put exhaust in the intake charge to reduce nitrous oxides, but that reduces peak combustion temperatures and the available power from the fuel charge. Running exhaust through the manifold also made the intake charge hotter, reducing efficiency. Old cars had fixed valve timing, which determines the relationship between power at high rpm and torque at low rpm. Some manufactures today have adjustment on the exhaust valve timing, and some even adjust both the exhaust and the intake timing to increase efficiency and torque at low RPM while changing valve overlap for high rpm horsepower with a net smaller displacement.

    Some modern cars still include EGR, but the catalytic converter technology is much better and more attention to exhaust system design has helped increase MPG.

    The actual energy used by the valvetrain has also been reduced by reducing the mass of the components in the valve train. Using a cog belt to a cam riding right above the valves reduces the energy used to activate the valves. That can really add up over the life of the engine.

    Another development was the inclusion of the electric cooling fan. Belt driven fans consumed huge amounts of power from the engine, even if they had a thermal clutch. Flex fans were supposed to reduce energy, but we are still talkin' in the multi-horsepower range just to move air across the radiator. The modern cooling fan uses anywhere from 9 to 20 amps at 12-14 volts. So worst case, it consumes 280 watts. Assuming alternator efficiency of 50%, running the fan uses 560 watts from the engine, which is still less than one horsepower. Furthermore, the electric fan rarely runs when the car is moving, hence saving more energy. Also to increase cooling system efficiency, they increased the cooling system pressure and temperature. Typical system temps. can reach 230F in today's cars. Older cars had 5 to 7lb raditor caps, running at not much over 170F. If you have the same size radiator with the same air speed and quantity to cool it, the hotter radiator will release MORE heat. It's hot--somebody please turn on the AC.

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  • happy
    Lv 5
    4 years ago

    they don't , 21 mpg today is still 21 mpg yesterday

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