Introduction

Abstract

The book is on design of diesel engines. The engine design is motivated by advanced emission standards, and yearning for fuel economy as well as downsizing to achieve light weight. The book develops the subject gradually by discussing various topics so that it is useful to a practising engineer and a student intending to develop a career in engine design. The book is broadly divided into five parts after introduction. The introduction chapter summarises the book after giving some thoughts on estimating the power, maximum torque and shape of the torque curve to satisfy vehicles on- and off-road.

1.1 Improved Diesel, to Ban Or Not to Ban

The diesel engine replaced the steam engine as the work horse of modern civilization for the past hundred years. It has been powering almost every hard-working machinery like ships, trains, agricultural tractors, buses and for the last fifty years passenger cars. The advantage of very high fuel efficiency, high back up torque enabled very wide application. The engine could accept widely varying quality of fuels. However, the stigma attached to the engine, of nitric oxides (NOx) and high particulate matter (PM) emission by virtue of smoke and unburnt oil as well as noise, could not be easily erased from the attention of ever vigilant society. It appeared the end of the road had been announced for the diesel engine: an old-fashioned technology and the root cause of disproportionate levels of pollution in cities and countryside, giving gave credibility to the headlines and commentary in the media. A number of improvements have taken place in the last twenty-five years to improve these difficulties successfully. Most importantly, diesel offers the efficient and flexible use of fuels with high energy density, and excellent storage and distribution options. A mix of technologies is needed in the future as the role the modern diesel engine has to play and hence, innovation instead of bans is the right direction (Anton Andres, Daimler 2018). The engine manufacturers are clearly aware of the responsibility towards climate, clean air as well as the demands of individual mobility:

• The advantage in respect to CO₂ over comparable gasoline engines is undisputed.

• The NOx emissions of many vehicles on the road reduced by up to 80%, through software updates relatively easily because of the rich experience and knowledge about the selective catalytic reduction (SCR).

• The new generation engines satisfying Euro-6 standards are having a huge market traction and the emissions are already far less than mandated limit values.

• In countries with strict emission control, the NOx pollution has dropped by more than 70% from 1990, even though the real driving cycle emissions (RDE) are higher the laboratory measurements, and in the next five years the drop is expected to be 90% from today.

• In cities like Delhi and Stuttgart it has been found the contribution by automobiles to PM is about 5% only. PM10 and PM2.5 from engines are drastically reduced with the introduction of Euro-6 norms with a Diesel Particulate Filter (DPF).

• With the new generation fuel injection systems working in conjunction with SCR and mild-hybrid options like start-stop, e-turbocharger offer substantial improvement to fuel consumption as well as emissions.

To do away with the diesel at this point in time would be a big mistake, for both environmental and economic reasons (Anton Andres, Daimler 2018). It may not be out of place to remind ourselves that reduction in well-to-wheel CO₂ emission is possible in case of electrical vehicles, only when the electricity is generated using non-coal-based power plants (Hofacker 2017).

1.2 Application of Diesel Engines

Diesel engines are developed in laboratories to satisfy emission standards, fuel efficiency demand and cost. Application of right diesel engines to the end application like a vehicle is important. Right choice of the size, power rating, weight and other parameters at the right cost decides the success of the engine in the market. Front loading the work to the simulation community is expected to give some relief to the designers and reduce the anxiety (Fancher 1979; Northcote 2006).

1.3 Duty of Diesel and Its Sizing

Interestingly one size of diesel does not fit all the nations, conditions and users. Whereas 400 hp-650 hp may be common for trucks in Europe with fast and safe roads at an average speed of 100 km/h, in an emerging county like India an average 50 km/h for a truck is quite common. Similarly, off road the rating is determined the initial expenditure as well as the size of the land holding. Therefore, the rating of the engine is nearly half in India since the speed or the land area determine the power. In addition, the average duty of a truck engine itself is about 50% in developed countries with the result of large excess power available for overtaking even at 150 km/h.

1.4 Notes on Weight, Fuel Consumption and Optimum Power for a Given GVW

1.4.1 Fuel Consumption

For pulling 50 tonnes at 60 km/h 152 kW (max) are needed. For pulling it at 80 km/h you need 225 kW (max), Fig. 1.1.

Fig. 1.1

Power demand from the prime mover as a function of road speed for different Gross Vehicle Weight (GVW)

We can estimate the fuel consumption as given in Table 1.1.

Table 1.1 kW, kmph, hours to travel 1000 km and energy required

Therefore, the increase in fuel consumption for achieving higher velocity alone is 12.5%. Higher speed and hence higher rating therefore do not pay in terms of fuel consumption, unless the vehicle power train is 12.5% (equivalent to an improvement in specific fuel consumption, SFC by 25 g/kW h) more efficient when scaled up. The fuel consumption of engines improves with brake mean effective pressure (bmep). However, the difference between bmeps 14 and 20 bar is about 1.5–2%. For example, the minimum SFC of 5.7 l Euro-4 is 206 g/kW h and 8 l Euro-4 is 203 g/kW h rated at 204 hp at 2100 rpm (15.3 bar bmep) and 306 hp at 1750 rpm (19.7 bar bmep) respectively corresponding to the optimum piston speed of 8 m/s for fuel consumption.

A plot is made of the energy consumption against rated power, kW Fig. 1.2, for different gross vehicle weight, GVW.

Fig. 1.2

Energy to travel 1000 km as a function of rated power for minimum time of travel

• Fuel consumption is minimum at around 50 km/h,

• Energy required to travel 1000 km at 3 different speeds namely 50, 70 and 90 kmph for different tonnages is plotted in dotted lines.

• Speeds in the range of 50 and 60 kmph offer minimum fuel consumption.

Figure 1.2 is plotted differently in Fig. 1.3 showing the penalty in fuel consumption if the speed is different from 50 kmph for different GVW. Thus, the optimum range of power required for minimum fuel consumption can be found, for example when the penalty is 1%, Fig. 1.4. This is shown in Table 1.2. In other words, rating high power engines at lower power can lead to improvement in fuel economy.

Fig. 1.3

Penalty in fuel consumption if the speed is different from 50 kmph as a function of road speed

Fig. 1.4

Relation of minimum and maximum of the range of optimum power rating against GVW (t) for a penalty of 1% with respect to the gross vehicle weight

Table 1.2 Optimum range in power for minimum fuel consumption

For grading and acceleration, some reserve has to be maintained. Usually, a 20% reserve is sufficient and this pushes the maximum speed capability to 75 kmph on level road at full load. Such an approach is used in most of the successful vehicles (Fig. 1.4) in India where speeds above 80 km/h at full load are not desired. In Europe, on the other hands, customers wish 120 kmph at the maximum and deviate substantially from the power required to run at 50–60 camphor example, a 35 tonner would be successful in India using 120 kW (160 hp) engine for reasons of fuel economy, whereas a 16 tonner would be designed with 240 kW (340 hp) in Europe to achieve the maximum speed of 140 kmph on level ground.

1.5 Typical Design Parameters

Piston speed

Type	Cylinders	Litre	Speed, rpm	Power, hp	Bmep, bar	Mass balancer, kg	Mass of EGR circuit
C	4	5.3	2170	306	23.8	13	13
	6	8	2170	459	23.8	0	20
A, B	4	3.8	2200	153	16.5	9	0
	6	5.7	2200	230	16.5	0	0

1. ^aAll are turbocharged intercooled engine

The mean piston speeds of some four and six-cylinder engines given in Table 1.3, are plotted in Fig. 1.5. The economic speed for best fuel consumption for these diesel engines is 8 m/s. Taking advantage of turbocharging, it is usual to design the modern engines at as low operating speed of the piston as possible. Nine m/s is usually the mean piston speed at the rated speed.

Table 1.3 Empirical formula

Fig. 1.5

Mean piston speeds at rated speed for different engines listed in the table

Maximum cylinder pressures

Figures 1.6 and 1.7 show the cylinder pressure of different engines estimated empirically (see example calculation below) for the engines listed in Table 1.3.

Fig. 1.6

Cylinder pressures, bmep and maximum pressure capabilities for SCR engines: B4 (4-cylinder, 3.7 l), B6 (6 cylinder, 5.7 l)

Example: Calculation of maximum cylinder pressure

kW	Given	165
hp	hp = kW/0.735	224
rpm	Given	2500
Litre	Given	5.7
Power/litre		28.9
Bmep at rated speed	900 × hp/(rpm × litre)	14.2
Boost	Bmep/5.5	2.6
Motoring pressure	compression ratio1.4 × boost	136.1
Peak pressure SCR Euro-4 or Euro-3, bar	1.05 × motoring pressure	142.9
Peak pressure 15% EGR, bar	1.15 × peak pressure SCR	164.4
Peak pressure 30% EGR, bar	1.30 × peak pressure 30% EGR	185.8

Weights

An empirical model for calculating Weight of engines depending on bore and peak pressure capability can be arrived at as explained below.

The predicted weight of vehicles in Fig. 1.8 using the formula seems to be working out within 2% accuracy for the six types of engines, Fig. 1.9.

Fig. 1.7

Cylinder pressures, bmep and maximum pressure capabilities for EGR engines: C4 (4cylinder 5.3 l), C6 (6-cylinder 8-litre)

Fig. 1.8

Ratio of weight of power train to maximum cylinder pressure of different vehicles

Fig. 1.9

Comparison of predicted weight and actual weight of engines

Importance of backup torque

For a given vehicle, the maximum road speed is a function of only the rated power. For acceleration, it is the backup torque or the maximum torque is important. For appreciating this Fig. 1.10 is useful.

Fig. 1.10

The process path when an engine is accelerated from speed A to B for an external resistance which increases parabolically with engine speed (obtained from vehicle speed)

Here, let T_r be the engine friction when declutched and vehicle reaction at the wheel with friction added when the vehicle is moving. Similarly, I be the engine inertia alone when declutched and the reflected vehicle inertia is added when the vehicle is moving. The maximum engine torque is shown by the black continuous line and the friction is shown by green line. When the engine is accelerated from low idle to high idle speed, for example, from point A to point B, the path taken by the engine is as shown by the dashed line. It can be seen that the excess torque given by (T_e − T_r) contributes to acceleration. Applying Newton’ second law,

$$ Acceleration = \left( {T_{e} - T_{r} } \right)/I $$

$$ time\;taken\;to\;accelerate\;to\;final\;speed,B \propto I/\left( {T_{e} - T_{r} } \right) $$

This model is validated over a number of similar agricultural tractors with the bare engine rated in the range 55–80 hp, Fig. 1.11. Assuming the friction torque is a small fraction of the engine torque, the acceleration time is inversely proportional to torque at low end. In Fig. 1.12, the Fig. 1.11 is plotted differently, to show the excellent correlation.

Fig. 1.11

Time to accelerate a tractor, for different settings of torque at low speed end

Fig. 1.12

Correlation of acceleration time (Fig. 1.11) with respect to the reciprocal of maximum engine torque at low speed end

System stability

The operating point of a diesel engine is determined by the solution of the curves of the engine torque versus speed and the resisting load torque versus engine speed. The operating point is stable only when the engine curve is having a negative slope and the resistance curve is positively sloped at the operating point. Only then, any disturbance increasing the speed will result in a net braking torque to bring the engine to the solution point; similarly, any decrease in speed will be restored by a net accelerating torque. Therefore, the engine should operate as in Fig. 1.13a and not as in Fig. 1.13b. Engine governing is designed to avoid the latter (Fig. 1.13b). Along the maximum torque line, it may not be always avoidable especially where the manifold pressure compensator cuts in.

Fig. 1.13

a Stable operation, b unstable operation

Manifold pressure compensation

A single stage turbocharger is not able to pump enough air in the low speed range into the engine. This becomes critical when the rated bmep of the engine is very high. Therefore, a manifold pressure compensation is integrated with the fuel injection system to reduce fuelling at low speeds to the level of a naturally aspirated engine. By this, in a small speed range, the torque drops precipitously as the speed decreases. The dropped torque is nearly equal to the maximum torque a naturally aspirated engine would produce. Now, when the vehicle is accelerated under load following a typical saw-tooth shaped curve of engine speed with respect to time; during the gear change after declutching, the engine speed drops naturally due to internal friction. The manifold pressure compensator would reduce the fuelling and hence the torque if the speed falls below the critical speed and the engine would hesitate to accelerate. Solutions are arrived at by using e-turbocharger or R2S turbocharger to avoid the severe drop in torque in the range of speeds envisaged during gear change.

1.6 Secrets of Fuel Economy

Even with the best designed and developed engine the important parameter of fuel economy may elude the vehicle customer for many reasons. Tyres have the highest influence on in fuel economy below 80 km/h, whereas aerodynamics is the most important factor above 80 km/I. The “rock solid” rules are listed in the table below (Guide, Cummins MPG 2003; van Dam et al. 2009):

Cause	% change in fuel consumption
2% reduction in aerodynamic drag	−1
Each 2 km/h increase, above 90 km/h	+2
Worn tyres	−7
Ribbed tyres	−3
Efficient driver	−20
Avoiding idle time of one hour in long haul	−1
Running in ~50000 km	–
Well formulated oil	−2

During service, the cause of excessive fuel consumption could be due to a number of factors (Guide, Cummins MPG 2003; Sturm and Hausberger 2005):

• Engine factors

• Vehicle factors and specifications

• Environmental factors

• Driver technique and operating practices

• Fuel system factors

• Low power or driveability problems