A combustion engine can work in a vacuum, providing an oxidizer. Without an oxidizer, the engine would be unable to run. In such a situation, a direct liquid oxygen injector would be used in place of the intake system. This would allow the valvetrain to remain essentially unchanged.
A combustion engine works by compressing gas inside the cylinder. In a gas engine, the gas volume is one liter (1000 cm3), and the uncompressed gas volume is 0.001 m3. This gas has five degrees of freedom and a compression ratio of 10 to one.
The entire combustion process requires a finite amount of time and volume. This volume is expressed by a ratio called L*: the pressure in the combustion chamber divided by the surrounding atmospheric pressure, or L*. In large engines, the balance is low, and smaller engines use a high contraction ratio. This allows for the same vaporization and dwells time but with a smaller volume.
Using the above example, a 300 rpm piston will complete the P-V cycle in 0.2 seconds. The pressure increase in this example is more than 10:1 and indicates an adiabatic gas compression. This process also increases the gas’s internal energy, which manifests as an increase in temperature and additional pressure.
The adiabatic process is of great importance in thermodynamics. It is derived from the Greek word adiabatic, which means “impassable wall.” It provides a direct relationship between work and heat. It’s also helpful in calculating the behavior of systems.
Another example is the asymmetric pressure distribution seen in a combustion chamber with a nozzle. The pressure at the bottom of the room is slightly lower than that at the top. Because of this, the force due to gas pressure in the chamber’s lower portion does not compensate for this external pressure difference. The resultant force pushes the section upward.
During the compression stroke, the trapped air-fuel mixture is compressed. Because of this, the air-fuel combination is hotter, which releases more energy when ignited. To achieve this, the intake and exhaust valves must close. The cylinder must also be sealed.
When the engine is running, the compression ratio is between 1.3.5. The -RC term reduces the VE by 1.3% to 3.5%. The machine is, therefore, less efficient than its non-adiabatic counterpart.
You should investigate the source if your engine has a low idle vacuum. One of the most common causes of low idle vacuum is a worn or damaged valve boot or manifold. Other causes include a blown head gasket, worn valve guides, or an improper idle mixture. Use a vacuum gauge to determine if your engine has a low vacuum.
Idle vacuum is measured in inches-Hg. A higher vacuum indicates that the engine requires less fuel and air. A low vacuum suggests that the valves are tight. An idle vacuum of less than ten inches-Hg may indicate a closed valve or intake leak. To check the vacuum level of your engine, consult the vehicle’s service manual.
Excess vacuum in the combustion chamber results in high operating costs and shortened engine life. It also dilutes the lubricating oil and renders it incapable of supporting combustion. When this happens, gasoline and oil mix, forming harmful gums and carbon deposits. This causes a decrease in engine performance and higher fuel bills.
Vacuum leaks in a car engine are the most common cause of poor idle performance. A damaged vacuum hose is the most obvious culprit, but a damaged intake manifold is another common culprit. Vacuum leaks in a car’s engine can also result from improper ignition timing.
Idle vacuum can be measured through a vacuum gauge. A pressure differential must be between the car’s engine and the air intake manifold. This will reduce the pressure in the intake manifold and the amount of vacuum. It is best to adjust the pressure differential to a level that will not disrupt the system’s operation. The pressure differential in a car engine should be less than the amount of vacuum the car needs to run at idle.
The idle vacuum in a car engine should be between 14 and 22 in. Hg. Increasing the vacuum can increase the engine’s efficiency. It’s analogous to how the lungs work. You can perform strenuous activities with a vital lung, while a weak lung quickly makes you out of breath.
The efficiency of a combustion engine in a vacuum
The efficiency of a combustion engine depends on the amount of air and fuel in the combustion chamber. However, it is essential to understand that the amount of air and power in a combustion chamber differs from the pressure outside the engine. This pressure difference will affect the air/fuel ratio.
High-speed exhaust gases, containing high temperatures and pressures, require a large section ratio. The section ratio is the difference between the pressure inside the combustion chamber and outside at the nozzle’s exit. The ratio is measured in pounds, kilograms, or Newtons. The pressure ratio between the two is called the section ratio, and a nozzle with this ratio is suited for high-speed operation.
Thermal efficiency in a combustion engine is roughly equal to the mass flow rate ratio. But the mechanical efficiency is higher than the thermal efficiency. The combustion temperature and the number of unburned hydrocarbons must be balanced against the increased amount of nitrogen oxides. Fortunately, there are several ways to improve the thermal efficiency of combustion engines:
A combustion engine in a vacuum has lower fuel efficiency than an engine in a familiar atmosphere. As the atmosphere contains only 21% oxygen, any fuel added will not burn completely. In addition, the energy that is too rich for the air will produce hydrocarbon pollutants and reduce the engine’s power.
To increase the efficiency of a combustion engine in a vacuum situation, the engine pumping piston has to work more than it would when there is no vacuum. A healthy engine produces a vacuum of about seventeen to twenty inches of mercury at idle. If the motor is not making this vacuum, it indicates worn components in the engine.
A combustion engine in a vacuum can run if an oxidizer is supplied. Without it, the machine wouldn’t work. It might also be possible to retain current valvetrain designs while using an oxidizer injector in a plenum. This would be an innovative solution for the engine.
Using a vacuum gauge to troubleshoot a leak
Vacuum is fundamental to internal combustion engines and helps run critical subsystems. This vacuum is generated by the engine’s pistons when they draw air and fuel into the cylinder. Suppose a vacuum leak is detected; philosophy leaks into the combustion chamber. This can be caused by various issues, including faulty gaskets, cracked hoses, and broken components.
Using a vacuum gauge to troubleshoot a leak in a combustion engine can be extremely helpful when determining the location of a problem. Because the vacuum gauge works in the machine, it can identify a leak or other issue in a matter of minutes. For example, if the vacuum gauge reveals an increased pressure when the throttle is opened, the problem may be in the intake valve or intake plenum.
Using a vacuum gauge to troubleshoot a leak in a combustion engine is particularly important when you suspect a valve or intake gasket leak. If the reading fluctuates between zero and two inches, the issue is likely with the valves. Vacuum readings in the engine’s intake manifold should equal the vacuum in the exhaust system at idle. Vacuum fluctuations can also indicate worn or damaged valve guides or valve springs.
While the vacuum gauge is not a foolproof method, it does provide an easy way to identify the source of a mechanical problem. Using it along with a vehicle’s service manual can help the repair technician pinpoint specific issues. Despite its limitations, the vacuum gauge remains an essential tool for professional mechanics and DIYers.
While vacuum leaks are often difficult to detect, they can make your vehicle suffer significant performance problems. The most common culprits are vacuum hoses and damaged intake manifolds. The latter, however, can be more challenging to diagnose.
A simple tool to troubleshoot a vacuum leak is a spray bottle full of soapy water. You can also use a smoke machine, a particular type of vacuum tester. When the engine is off, pump up the vacuum lines and measure the pressure. If you notice a pressure drop, the leak has likely occurred.