- 2068 Bhadra · 6 marks
Air standard cycle; diesel vs Otto
Short answer
Air standard cycle: air as ideal gas with constant specific heats, reversible processes, external heat addition/rejection. Otto cycle adds heat at constant volume, η=1−1/r^(γ−1); Diesel adds heat at constant pressure, η=1−(rc^γ−1)/[γ r^(γ−1)(rc−1)]. Diesel engines use higher compression ratios (14-22) than Otto (6-10), giving higher real efficiency.
What is air standard cycle? Differentiate between diesel cycle and otto cycle.
Air standard cycle
An air standard cycle is an idealised cycle for an internal combustion engine or gas turbine in which air is the working fluid and the following assumptions hold:
- A fixed mass of air behaves as an ideal gas with constant specific heats.
- All processes are internally reversible.
- Combustion is replaced by heat addition from an external source, and the exhaust by heat rejection to a sink that returns the air to its initial state.
- Kinetic and potential energy changes are neglected.
Examples: Otto, Diesel, Dual and Brayton cycles.
Diesel cycle versus Otto cycle
| Point | Otto cycle | Diesel cycle |
|---|---|---|
| Engine | Spark ignition (petrol) | Compression ignition (diesel) |
| Processes | 2 isentropic + 2 constant volume | 2 isentropic + 1 constant pressure + 1 constant volume |
| Heat addition | At constant volume | At constant pressure |
| Compression ratio | 6-10 | 14-22 |
| Efficiency | η = 1 − 1/r^(γ−1) | η = 1 − (1/r^(γ−1)) (r_c^γ − 1)/(γ(r_c − 1)) |
| Efficiency for same r | Higher | Lower |
| Depends on | r only | r and cut-off ratio r_c |
| Ignition | Spark plug | Compression heat |
| Mean effective pressure | Higher for the same r and heat input | Lower for the same r and heat input |
| Peak pressure | Higher for the same r (sudden heat release at constant volume) | Lower for the same r |
For the same compression ratio the Otto cycle is more efficient, but the diesel engine can use a much higher compression ratio, so in practice it gives higher efficiency.
✓ Answer checked by Claude Opus 5.5 (high effort)
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