Which engine is more powerful Diesel or petrol?

First of all let me make one thing clear that when an engine is stated to have a specific power or torque, these are the peak values on the power or torque curves. These are just one point on the respective curves and basically mean that when engine will run at that specific RPM, it will produce maximum power or torque, as the case may be. At any other RPM, power and torque will be less than this stated peak value.

The graph below represents an engine with 110 kW power at 5500 RPM and 236 NM of torque at 3600 RPM. At other RPMs (which is the case most of the time when driving), both the power and torque are much less than peak values. This is true for all types of engines.

Hence, an engine which has flatter Torque and Power curves will perform much better over a wider RPM range. This type of engine will be more responsive and powerful than an engine which has steep power and torque curves.

Now coming to the question i.e. Which engine is more powerful Diesel or Petrol? For engines of same size (CC), petrol engine will produce more Power than diesel engine. This is because petrol engines burns more fuel than diesel engine and also are compact and have smaller stroke length, and hence they operate at higher RPM than diesel engines.

Because of higher compression ratio of diesel engine, it has longer stroke length, and hence lower RPM but higher Torque than a petrol engine at same RPM. Power, Torque and Speed have following relationship:

Power (HP) = Torque (lbs-ft) x Speed (RPM) / 5252

Power (kW) = Torque (N.m) x Speed (RPM) / 9.5488

Petrol engine will have higher power than diesel engine because it can run at much higher RPM and as power is directly proportional to the RPM hence, petrol engine will produce more power, although diesel engine has more Torque, but it will not produce same amount of power as petrol engine, as it has low RPM.

Coming to actual figures obtained from production vehicle engines, Maruti Suzuki Swift is available with both petrol and diesel engine options - a 1197cc four-cylinder petrol engine and a 1248cc four-cylinder diesel engine. The petrol engine produces 83bhp peak power, which is higher than the 75bhp peak power output of the diesel engine. However, the diesel engine produces 190Nm of peak torque, 77Nm more than the 113Nm peak torque output of the petrol engine.

Concluding all this power and torque talk, I would like to clarify one thing that Torque at the wheels can be increased by increasing the gear ratio and hence, it is not an intrinsic property of an engine, while power is intrinsic to the engine and cannot be increased. Hence, if you have a more powerful engine, you can get more torque at the wheels by manipulating the gear ratios but you can do nothing to increase the power of your engine except to modify or replace it.

EDIT 1:

Upon request from readers, for comparison purposes, I am adding Torque and Power Curves for similar size (cc) Petrol and Diesel Engines:

Mitsubishi Pajero Petrol 3.5L V6 Engine:

Peak Power = 217 HP @5500 RPM, Max Torque = 257 lb-ft @3750 RPM

Mitsubishi Pajero Diesel 3.2L DI-D VR-II Engine:

Peak Power = 188 HP @3500 RPM, Max Torque = 325 lb-ft @2000 RPM

It can be seen that Petrol Engine has more power while Diesel Engine has more Torque. Moreover, Petrol Engine can operate at higher RPM than Diesel Engine.