# Calculating engine parameters from power requirement

• Automotive
• shashanksuresh91
In summary, the conversation discusses determining engine power requirements based on driving dynamics and how to extend this to engine parameters such as bore, stroke, compression ratio, and fuel injection. It also mentions the use of air fuel ratio and provides examples of engine performance for different types of vehicles. Additionally, a resource for calculating engine horsepower is mentioned.

#### shashanksuresh91

Hi,
I've defined engine power requirements based on driving dynamics, but am not sure how to extend this to engine parameters. From design requirements, I've fixed my gear ratios, and hence can determine how much power and torque is required from the engine at a particular speed.
I want to then determine the architecture of the engine, in terms of bore, stroke, compression ratio, amount of fuel injected, as values that I can tune. Since its an SI engine, I'm taking air fuel ratio as stoichiometric, but I'm still unable to figure out how to determine the engine parameters as a function of the power required ?

shashanksuresh91
As a general rule, most engines with 4 valves per cylinder will produce about 65 to 85 ft-lbs per liter. The 85 ft-lb figure is common for performance motorcycles and high end exotic type cars. For a mid-range example, a 2015 Genesis 5.0 normally aspirated v8 has a 5.038 liter engine, produces 383 ft lb of torque for 76 ft lbs per liter at 5000 rpm, and it produces 420 hp at 6000 rpm. The power is an issue of how high the rpm is at peak power. For a high end example, Kawasaki ZX14R motorcycle, 1.441 liter engine, 120 ft lb of torque at 7600 rpm for 83.3 ft lb torque per liter, and produces 197 hp at 10,000 rpm.

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If you want to see the "performance envelope" it's easier to use the torque curve. The power curve produces different acceleration rates in each gear, and acceleration rate per unit power declines as engine rpm and/or drive wheel rpm increases. Note that drive wheel power is the same as engine power less driveline losses regardless of gear ratio.

## 1. How do you calculate engine power requirements?

To calculate engine power requirements, you need to know the desired speed and torque of the engine. Then, you can use the equation P = (T x N) / 5252, where P is power in horsepower, T is torque in pound-feet, and N is rotational speed in RPM.

## 2. What is the relationship between power and torque in an engine?

Power and torque are directly proportional in an engine. This means that as power increases, torque also increases. However, the relationship between the two can vary depending on the engine design and operating conditions.

## 3. How does engine size affect power requirements?

Generally, a larger engine size will have a higher power output compared to a smaller engine. However, other factors such as engine design and efficiency can also impact the power requirements.

## 4. What is the role of fuel efficiency in calculating engine parameters?

Fuel efficiency plays a crucial role in calculating engine parameters, as it affects the amount of power that can be produced from a certain amount of fuel. A more efficient engine will require less fuel to produce the same amount of power, resulting in lower power requirements.

## 5. Are there any other factors that can impact engine power requirements?

Yes, there are several other factors that can impact engine power requirements, such as altitude, temperature, and air density. These factors can affect the amount of oxygen available for combustion, which can impact the engine's power output.