Modern Engineering Thermodynamics

514 CHAPTER 13: Vapor and Gas Power Cycles
and, for the isobaric process from 4s to 1, we have
T 1 /T 4s = v 1 /v 4s = CO
where CO is called the cutoff ratio of the engine. For the isochoric process from 2s to 3, it can easily be shown
that T 2s /T 3 = CO k , then
ðη T Þ Diesel
cold ASC
= 1 − CR1−k ðCO k − 1Þ
kðCO − 1Þ
(13.36)
WHAT DOES CUTOFF MEAN?
The term cutoff is another archaic steam engine jargon term that has been absorbed into modern IC engine terminology.
It was introduced in the 1780s by James Watt, when he realized that, if the steam entering the cylinder was “cut off”
(i.e., shut off) when the piston had completed only a portion of its stroke and the natural expansion of the steam was
allowed to complete the stroke, then the engine’s thermal efficiency increased significantly. Today, this term is used to
indicate where the combustion process “cuts off” (i.e., stops) in a compression ignition internal combustion engine. It is
determined by the geometry of the combustion chamber and the fuel charge.
EXAMPLE 13.17
The Wärtsilä RT-flex96C, the two-stroke, turbocharged, low-speed Diesel engine shown in Figure 13.54, was manufactured by
the Finnish manufacturer Wärtsilä. With 14 cylinders, it is one of the largest reciprocating engines in the world. Determine
the following items for the Diesel cold ASC with k = 1.4. Assume that, when the engine is producing 80,080 kW of output
power, it has a compression ratio of 18.0 to 1 with a cutoff ratio of 2.32, and it uses fuel with a heating value of 45.5 × 10 3
kJ/kg with a fuel flow rate of rate 3.35 kg/s.
a. The Diesel cold ASC thermal efficiency of the engine.
b. The actual thermal efficiency of the engine.
c. The mechanical efficiency of the engine.
FIGURE 13.54
Example 13.17.
The 14-cylinder Wärtsilä RT-flex96C marine engine was put into service in September 2006 aboard the Emma Mærsk. Its
maximum continuous power output was 80,080 kW (108,920 bhp) at 102 rpm. Measuring 27.3 m long and 13.5 m high,
its overall weight is 2300 tons.