Otto vs Diesel vs Dual Cycle: Which Engine Cycle Wins?

If you've ever wondered why diesel engines feel like tanks while petrol engines feel zippy, the answer lies partly in thermodynamics — specifically, in how the Otto, Diesel, and Dual cycles stack up against each other.

These three air-standard cycles model how internal combustion engines theoretically work. Comparing them isn't as simple as saying "X is always better than Y" — it depends entirely on what condition you hold constant while comparing. Change the constraint, and the winner changes too. Let's break down the three classic comparisons.

A Quick Refresher

All three cycles share the same first step: an isentropic (reversible, no heat loss) compression, labeled process 1–2. Where they diverge is in how heat gets added during combustion:

• Otto cycle — heat added at constant volume (think: spark-ignition petrol engines)
• Diesel cycle — heat added at constant pressure (think: compression-ignition diesel engines)
• Dual cycle — a hybrid, adding heat partly at constant volume and partly at constant pressure (a more realistic model of real diesel engines)

With that in mind, here's how they compare under three different scenarios.

Round 1: Same Compression Ratio, Same Heat Input

Picture all three cycles drawn on the same P-V diagram, starting from the same point with the same compression ratio. Since the compression process is identical, the cycles only start to differ once heat addition begins.

Here's the interesting part: if you pump in the same amount of heat into each cycle, the temperature spike you get is different for each:

$$T_{Otto} > T_{Dual} > T_{Diesel}$$

The Otto cycle, adding heat instantaneously at constant volume, shoots temperature up the fastest. Since the net work output of a cycle is proportional to the area it encloses on the P-V diagram, this translates directly to:

$$W_{Otto} > W_{Dual} > W_{Diesel}$$

The verdict: At the same compression ratio and same heat input,

$$\eta_{Otto} > \eta_{Dual} > \eta_{Diesel}$$

Otto wins this round, hands down.

Round 2: Same Maximum Pressure, Same Heat Input

Now change the rules. Instead of fixing the compression ratio, fix the maximum pressure each cycle reaches — so points 3, 3′, and 3″ (the end of heat addition) all sit on the same pressure line.

With equal heat supplied across all three, the deciding factor becomes how much heat each cycle rejects afterward. It turns out:

• Otto rejects the most heat
• Diesel rejects the least
• Dual sits in the middle

Recall the efficiency formula:

$$\eta_{thermal} = 1 - \frac{Q_R}{Q_s}$$

Since heat supplied ($Q_s$) is the same for all, whoever rejects less heat ($Q_R$) wins on efficiency.

The verdict: At the same maximum pressure and heat input,

$$\eta_{Diesel} > \eta_{Dual} > \eta_{Otto}$$

The tables have completely turned — Diesel now comes out on top.

Round 3: Same Maximum Pressure, Same Maximum Temperature

One more scenario: fix both the maximum pressure and the maximum temperature across all three cycles. In this case, something neat happens — all three cycles reject the exact same amount of heat.

The difference now comes from how much heat each one needs to supply in the first place:

• Diesel needs the most heat input
• Otto needs the least
• Dual, again, sits in between

Using the same efficiency formula, with $Q_R$ now fixed, the cycle that supplies more heat for the same rejected heat ends up more efficient:

$$\eta_{Diesel} > \eta_{Dual} > \eta_{Otto}$$

Diesel wins again.

Putting It All Together

What's held constant	Efficiency ranking
Compression ratio + heat input	Otto > Dual > Diesel
Max pressure + heat input	Diesel > Dual > Otto
Max pressure + max temperature	Diesel > Dual > Otto

So Which Cycle Is Actually Better?

This is the part that trips people up. At the same compression ratio, the Otto cycle is theoretically more efficient. So why do real diesel engines get better mileage than petrol engines?

Because in practice, diesel engines aren't operating at the same compression ratio as petrol engines — they run at much higher compression ratios (often 18:1 to 22:1, versus 8:1 to 12:1 for petrol). And higher compression ratio means higher efficiency, full stop. So even though the Diesel cycle "loses" to Otto when compared at equal compression ratios, real-world diesel engines exploit their ability to run at far higher ratios — and that's where their efficiency advantage actually comes from.

It's a great reminder that in thermodynamics (and engineering generally), the answer to "which is better?" almost always comes with an asterisk: better, under what conditions?