Date: Sun, 25 Mar 2012 15:36:18 -0400
Reply-To: Kim Brennan <kimbrennan@MAC.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Kim Brennan <kimbrennan@MAC.COM>
Subject: Re: Fuel consumption in different gears - how does the energy /
fuel work?
In-Reply-To: <20120325141510.MQ82Q.140690.imail@eastrmwml214>
Content-type: text/plain; CHARSET=US-ASCII
It takes a certain minimum amount of fuel to fire a cylinder. For an engine sitting still not under load running at a constant rpm it should be obvious that one running at a higher rpm will consume more fuel than one running at a lower rpm. Less obvious is that an engine under load will consume more fuel than another at the same rpm that is not under load. As others have mentioned also an engine will run most efficiently at a certain rpm, usually at maximum torque.
Sent from my iPad
On Mar 25, 2012, at 2:15 PM, Dave Mcneely <mcneely4@COX.NET> wrote:
> ---- Kim Brennan <kimbrennan@MAC.COM> wrote:
>> You are confusing how much energy you need, versus how much energy you are USING.
>>
>> When you are using the lower gear, you are consuming more energy, which is getting lost as heat.
>
> Well, most of the energy burned is lost as heat, regardless of gear. I believe Roland's question recognizes that the vehicle is overall less efficient at the lower gear. He is asking why. Now, if you are saying that the lower gear itself is less efficient, there must be a mechanical reason for that.
>
> What about driving in the lower gear is less efficient?
>
> mcneely
>
>>
>>
>> On Mar 24, 2012, at 9:56 PM, Roland wrote:
>>
>>> So, in driving along, with the brain wondering as usual. How much more
>>> fuel is consumed in a lower gear vs higher gear at the same speed?
>>>
>>> I ran a quick observation. On level ground, at 45 mph, the RPMs in 4th
>>> gear were about 2400, and in 3rd gear it was about 3400. (These are not
>>> absolute numbers since there are some mods on my van, and your experience
>>> may be different).
>>>
>>> And being a fan of physics, and mechanics, and all things we'd like to
>>> understand better -- I was wondering, at this speed how much more fuel is
>>> consumed in 3rd gear vs 4th gear? Now the immediate answer is none! It
>>> takes the same energy to drive at 45 mph in 3rd or 4th. The wind
>>> resistance is the same, the drive-line loss is the same, other friction
>>> like tires are the same.
>>>
>>> It seems like the ECU is most likely properly calibrated to consider
>>> gearing (just through the senors). In 3rd gear, it would inject less fuel
>>> per combustion, since there are more combustion events, and the required
>>> amount for each combustion event is less. And in 4th gear, it injects more
>>> since there are fewer combustion events per time period, and so each
>>> combustion event is required to be stronger.
>>>
>>> And of course the relationship is degenerative since at any speed the
>>> engine must have a minimum amount of fuel, so this question may not apply
>>> to really low speeds or at idle.
>>>
>>> Again, the same energy is required to push the van at 45 mph in either 3rd
>>> or 4th. I assume that 3rd gear will use more fuel, but why?
>>>
>>> Thanks guys!
>>>
>>> Roland
>
> --
> David McNeely
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