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Tree Monkey MS462 - PART 2

tree monkey

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My Dad said that in the olden days, some car makers put vacuum gauges in their dashboards and called them "fuel economy gauges" ... more vacuum, higher fuel economy.

I know diesel engines don't have any vacuum, but gas engines ... even at full throttle, it seems to me they still have to have some vacuum, otherwise air wouldn't be drawn into the cylinder. And I bet if you put a vacuum gauge on the intake manifold, you'd still see at least some vacuum (i.e., pressure would be lower than local ambient pressure).

no vacuum in the intake at full throttle, no vacuum hoses on race cars, unless they add a vacuum pump
 

XP_Slinger

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no vacuum in the intake at full throttle, no vacuum hoses on race cars, unless they add a vacuum pump
Because the throttle plate is restricting ambient pressure entry at idle. Full throttle or no restriction on entry will still pull 1 to 2 inches of mercury...very very close to no vacuum.
 

RedGas

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If there were no pressure differential between ambient and inside the cylinder, air would never have gone into the cylinder except through Brownian motion.

Also, I suspect that there may be difficulties (due to the Bernoulli Principle) in measuring air that is moving at high velocity (as in an intake manifold). Normally a fluid or gas that is moving fast will be at a lower pressure than fluid or gas that isn't moving as fast (which is why airfoils and venturis work)...
 
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cus_deluxe

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Yes, I am nitpicking semantics and I have mentioned some reasons why making those distinctions are important, but you are now attempting to mix them. How/why did principles of thermodynamics get introduced when they have nothing to do with the elements (basic physics IMHO) supporting this discussion? Once the fire is about to be lit in the cylinder and the gaseous energy exchange processes commence we'll be talking thermodynamics... but those terms are irrelevant at this stage.
Enthalpy, in a nutshell, is the tendency toward disorder in the universe. Its why when you spray febreeze after taking a *s-word, a few minutes later you smell the febreeze elsewhere. Any time there is a potential difference (whether its pressure/temperature/concentration of a certain element/compound etc...) things move from an area of high potential to low potential (or in this case an area of positive pressure to an area of negative pressure). In the case of a two stroke engine, this potential is created by the movement of the piston, in the definition above the “work required to make room for it (the system- 2 stroke engine crankcase) by displacing its environment and establishing its volume and pressure.” I dont understand at all how it could be explained in a way where the piston was not directly responsible for the function of a 2 stroke engine.
 

CR888

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Motors are electric
Engines are gas

Hope I’m not coming across as a dick but your wrong

:D
Does this mean General Motors (GM) has secretly been making hybrids for about a century? Did steam engines have hidden gas tanks? Are motorbikes just the wrong term for a two wheeled gas engine powered bike? I'm confused now so I'll sit back. The strato subject has gotten so far off topic we ended up on the beach with the 'hoover lady'!!
 

cus_deluxe

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My Dad said that in the olden days, some car makers put vacuum gauges in their dashboards and called them "fuel economy gauges" ... more vacuum, higher fuel economy.

I know diesel engines don't have any vacuum, but gas engines ... even at full throttle, it seems to me they still have to have some vacuum, otherwise air wouldn't be drawn into the cylinder. And I bet if you put a vacuum gauge on the intake manifold, you'd still see at least some vacuum (i.e., pressure would be lower than local ambient pressure).
Yes theres always some vacuum in a gas engine at full throttle, but very close to zero. Diesels do not have a throttle body to create vacuum, they regulate power/rpm w fuel delivered rather than the air fuel mix in a gas engine. Ever seen a diesel engine run away? Pretty scary
 
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