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redpoint5 · 07-10-2016, 05:56 AM

The power and speed of regular gasoline engines is controlled by restricting air to the cylinders by varying the throttle, which is a valve that restricts airflow to the engine. Whenever the throttle is in any position other than fully open (floored), it is restricting air into the engine. This restriction causes a difference in pressure between the atmosphere (about 14.7 psi) and the combustion chamber as the intake stroke tries to draw air into the engine (a vacuum is created). The less the throttle is opened, the more vacuum there is (difference in pressure).

Visualize the engine as a syringe with no needle on it. As you pull the plunger back, a vacuum is created which is immediately filled by the surrounding atmospheric pressure. If you were to restrict air from entering the syringe by placing your finger over the opening, you would feel the resistance of the vacuum as you pull the plunger back. It takes extra effort to pull the plunger against the vacuum created by blocking the opening to the syringe.

In the same way, energy is lost in an engine when the piston intake stroke works against the throttle, which is partially blocking air from entering. High vacuum (not pressing the accelerator pedal much) is an inefficient way to run an engine since it is working to overcome the vacuum.

This is a big reason why powerful cars are not fuel efficient. If you have a 500 horsepower car, but only need 20 horsepower to steadily cruise down the street, the throttle is barely being opened to supply the needed power. The other 480 horsepower is being restricted by the vacuum created, and the engine is working against that huge vacuum.

This is also why the "pulse and glide" technique is good for fuel economy. It maximizes fuel economy by more fully opening the throttle valve, which reduces the vacuum that the engine has to work against. The vehicle accelerates during the efficient "pulse" phase, and then coasts in the "glide" phase.

As an aside, electric vehicles don't suffer such a huge efficiency penalty for being powerful. A 500 horsepower EV can be nearly as efficient as a 100 horsepower EV. No vacuums to overcome!

People refer to the accelerator as the gas pedal, but it is more accurately an air pedal. The fuel is just added in proportion to the amount of air that is allowed into the engine.

In summary; high vacuum is inefficient (accelerator not pressed very much), and low vacuum is efficient (accelerator almost all the way pressed). The 2 biggest reasons why we don't run an engine at the most efficient throttle opening at all times is that:

1. It's not safe to drive that fast
2. Efficiency drops as speed increases due to exponentially increasing wind resistance

07-10-2016, 05:56 AM	#3 (permalink)
redpoint5 Human Environmentalist Join Date: Aug 2010 Location: Oregon Posts: 12,815 Acura TSX - '06 Acura TSX 90 day: 24.19 mpg (US) Lafawnda - CBR600 - '01 Honda CBR600 F4i 90 day: 47.32 mpg (US) Big Yeller - Dodge/Cummins - '98 Dodge Ram 2500 base 90 day: 21.82 mpg (US) Chevy ZR-2 - '03 Chevrolet S10 ZR2 90 day: 17.14 mpg (US) Model Y - '24 Tesla Y LR AWD Pacifica Hybrid - '21 Chrysler Pacifica Hybrid 90 day: 43.3 mpg (US) Thanks: 4,327 Thanked 4,480 Times in 3,445 Posts	The power and speed of regular gasoline engines is controlled by restricting air to the cylinders by varying the throttle, which is a valve that restricts airflow to the engine. Whenever the throttle is in any position other than fully open (floored), it is restricting air into the engine. This restriction causes a difference in pressure between the atmosphere (about 14.7 psi) and the combustion chamber as the intake stroke tries to draw air into the engine (a vacuum is created). The less the throttle is opened, the more vacuum there is (difference in pressure). Visualize the engine as a syringe with no needle on it. As you pull the plunger back, a vacuum is created which is immediately filled by the surrounding atmospheric pressure. If you were to restrict air from entering the syringe by placing your finger over the opening, you would feel the resistance of the vacuum as you pull the plunger back. It takes extra effort to pull the plunger against the vacuum created by blocking the opening to the syringe. In the same way, energy is lost in an engine when the piston intake stroke works against the throttle, which is partially blocking air from entering. High vacuum (not pressing the accelerator pedal much) is an inefficient way to run an engine since it is working to overcome the vacuum. This is a big reason why powerful cars are not fuel efficient. If you have a 500 horsepower car, but only need 20 horsepower to steadily cruise down the street, the throttle is barely being opened to supply the needed power. The other 480 horsepower is being restricted by the vacuum created, and the engine is working against that huge vacuum. This is also why the "pulse and glide" technique is good for fuel economy. It maximizes fuel economy by more fully opening the throttle valve, which reduces the vacuum that the engine has to work against. The vehicle accelerates during the efficient "pulse" phase, and then coasts in the "glide" phase. As an aside, electric vehicles don't suffer such a huge efficiency penalty for being powerful. A 500 horsepower EV can be nearly as efficient as a 100 horsepower EV. No vacuums to overcome! People refer to the accelerator as the gas pedal, but it is more accurately an air pedal. The fuel is just added in proportion to the amount of air that is allowed into the engine. In summary; high vacuum is inefficient (accelerator not pressed very much), and low vacuum is efficient (accelerator almost all the way pressed). The 2 biggest reasons why we don't run an engine at the most efficient throttle opening at all times is that: 1. It's not safe to drive that fast 2. Efficiency drops as speed increases due to exponentially increasing wind resistance __________________ Gas and Electric Vehicle Cost of Ownership Calculator Give me absolute safety, or give me death! Last edited by redpoint5; 07-10-2016 at 06:27 AM..