Designing a Car Engine Piston: Thermodynamics Question

In summary: At 1500 rpm, the engine will be consuming 3.8 litres/100km. So your answer is within reasonable bounds.In summary, Chet designed a piston for a hypothetical car that uses octane as its fuel. He started by selecting the fuel, which according to some internet source (I forgot, can't cite) suggested that he should use Octane, so he wrote the chemical equation, . He then computed the densities of air and octane, and randomly selected a Vp = 1.8L volume, so he wrote these two equations, . From which, when he solved it, he got a total mass of 2.448 grams. Next, he started to figure out the heat of combustion and
  • #1
gezibash
24
0
Recently I have been studying thermodynamics and I wanted to analyze a fictitious car piston of my own design. Indeed I have quite some questions, but the most important is this one, and maybe someone could give me an opinion on my progress.

I started off by selecting the fuel, which according to some internet source (I forgot, can't cite) suggested that I should use Octane, so I wrote the chemical equation,

[tex] C_8H_{18} + 12.5(O_2+3.76N_2) \rightarrow 8CO_2 + 9H_2O + 23.5N_2 [/tex]
and I used the AFR ultimately computing the ratio to be,

[tex] i = \frac{m_o}{m_f} = 15.0279 [/tex]
After this, I computed the densities of air and octane,

[tex] \rho_A = 1.2754 \frac{\text{kg}}{\text{m}^3} [/tex]
and

[tex] \rho_F = 703 \frac{\text{kg}}{\text{m}^3} [/tex]
I randomly selected a Vp = 1.8L volume, so I went ahead and wrote these two equations,

[tex]
m_A = i \cdot m_F \\
\frac{1}{\rho_A}m_A + \frac{1}{\rho_F}m_F = V_p
[/tex]
From which, when you solve it, you get a total mass of

[tex]
m_T = m_A + m_F = 2.448 \; \text{grams}
[/tex]
My question is, is this feasible? Does the entire mass of the fuel and air mixture amount to about 2.448 grams in a single cylinder? If not, where did I go wrong?

Also, I would be most grateful if someone could point to a book or anything of the educational nature on this subject?

My next steps from here would be to try and figure the heat of combustion and then I will start to compute an Otto Cycle, perhaps later even use a more realistic intake/exhaust cycle.
 
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  • #2
2.5grams sounds OK.
Real engines do not operate at stoichiometric air fuel ratios, they have a massive excess of air.
 
  • #3
Thanks a lot. Yeah, I suppose I should fix that.
 
  • #4
Typical air fuel ratios are indeed 15. Show us how you calculated the densities of the fuel and the air. I hope you took into account the mole fractions of the gases in the mixture.

Chet
 
  • #5
Hi Chet, thanks for the reply. Sadly I did not manually calculate the densities, I used Tables from the internet to get them at 20°C, 1 bar - since I figured those would be intake conditions.
 
  • #6
gezibash said:
Hi Chet, thanks for the reply. Sadly I did not manually calculate the densities, I used Tables from the internet to get them at 20°C, 1 bar - since I figured those would be intake conditions.
For those conditions, I got 2.28 gm for an afr of 15.

I got mole fraction of air = partial pressure of air (atm) = 0.9833
mole fraction of fuel = partial pressure of fuel (atm) = 0.01667
mass of air = 2.136 gm
mass of fuel = 0.1424 gm

Chet
 
  • #7
Chestermiller said:
For those conditions, I got 2.28 gm for an afr of 15.

Yes, that seems pretty good. I was just making sure that the answer itself is logical, so anything around 2-3 grams would be on the right track here.
 
  • #8
A way of checking this is to figure out the fuel consumption rate. You can multiply by the number of cylinders and the engine speed to get the fuel consumption rate. Assume that, at cruising speed, the engine speed is about 1500 rpm. Does this make sense in terms of gas mileage?

Chet
 
  • #9
Yeah, but I would have to time the cylinder stroke. Check my train of thought here,

[tex] n_1\;m_1\;N_c = \dot{m} [/tex]
Where [itex]n_1 = 1500\;\text{rpm}[/itex], [itex]m_1 = 2.448\;\text{gm}[/itex] and [itex]N_c = 8[/itex] (number of cylinders).
 
  • #10
Lol I could not have been wronger than my previous post. I used the combined mass of both the fuel and air. Clearly that is not a fuel consumption rate. I need to use [itex]m_1\approx 0.15 \text{gm}[/itex]
 
  • #11
You also need to account for how often intakes occur. Presumably it's a four stroke engine...Not a mythical half stroke engine as your equation implies..The average V8 will cruise close to 1500rpm at highway speed (100km/hr)
With that information you can go ahead and convert your fuel mass flow rate to litres/100km (or mpg if you're that way inclined) so you can see how your values compare with real world fuel consumption data.
 

1. What is the purpose of a car engine piston?

The main purpose of a car engine piston is to convert the energy produced by the fuel combustion into mechanical energy, which powers the car's movement. It moves up and down inside the engine's cylinders, creating the necessary pressure to drive the car's crankshaft.

2. How does thermodynamics play a role in designing a car engine piston?

Thermodynamics is the study of heat and its relationship to energy. In the case of a car engine piston, thermodynamics is essential in determining the most efficient design for converting heat energy from the fuel combustion into mechanical energy. It also helps in optimizing the performance of the engine by balancing factors such as temperature, pressure, and volume.

3. What materials are commonly used in making car engine pistons?

The most commonly used materials for car engine pistons are aluminum alloys, cast iron, and steel. Aluminum alloys are popular because they are lightweight and have good thermal conductivity, while cast iron and steel are used for their strength and durability.

4. How are the dimensions of a car engine piston determined?

The dimensions of a car engine piston are determined based on the engine's displacement, compression ratio, and desired power output. The bore size, stroke length, and compression height are critical dimensions that must be carefully calculated to ensure optimal performance and efficiency of the engine.

5. What factors should be considered when designing a car engine piston?

When designing a car engine piston, factors such as thermal expansion, compression ratio, combustion pressure, and weight must be carefully considered. The piston must also be designed to withstand high levels of heat and stress to ensure its durability and longevity. Other factors to consider include cost, manufacturability, and compatibility with other engine components.

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