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Chapter 25
Effects of Leaked Exhaust System on Fuel
Consumption Rate of an Automobile
Peter Kayode Oke and Buliaminu Kareem
1 Introduction
A car exhaust system consists of a series of pipes that links the burnt exhaust gasses
in the engine cylinder through an exhaust manifold, catalytic converter, silencer, and
muffler to the atmosphere [1–4]. The exhaust systems consist of tubing, which are
used for discharging or expelling burnt gasses or steam through the help of a
controlled combustion taking place inside the engine cylinder [5,6]. The major
components used in a typical automobile exhaust system are: exhaust manifold,
resonator, catalytic converter, exhaust pipe, muffler, tail pipe, “Y” pipe and ball
flanges [7,8]. The products of combustion from internal combustion engines contain
several constituents that are considered hazardous to human health, including CO,
UHCs, NO
x
, and particulates (from diesel engines). In a bid to reduce the effects
caused by these gasses, exhaust system’s components are designed to provide
suitable and effective exhaust flow, reduction of noise and emission levels, and
conversion of the gasses to water vapor and carbon (iv) oxide at the exhaust [9,10].
The exhaust system is well designed to sustain engine performance. In an attempt to
reduce these emissions, several devices have been developed to arrest the dangerous
emissions [9]. A thermal reactor is seldom used to oxidize UHC and CO [2].
Catalytic converters utilize a catalyst, typically a noble metal such as platinum,
rhodium, or palladium, to promote reactions at lower temperatures. In all cases, an
arrangement which requires that the engine be operated with a rich mixture which
decreases fuel economy is emphasized.
P.K. Oke (*) • B. Kareem
Department of Mechanical Engineering, The Federal University of Technology,
P.M.B. 704, Akure, Ondo State, Nigeria
e-mail: okekayode@yahoo.com;karbil2002@yahoo.com
S.-I. Ao and L. Gelman (eds.), Electrical Engineering and Intelligent Systems,
Lecture Notes in Electrical Engineering 130, DOI 10.1007/978-1-4614-2317-1_25,
#Springer Science+Business Media, LLC 2013
301
It is worthy of note that, first, the exhaust gasses or moisture must be at or above
a certain temperature [1]. This is why the converter is placed close to the engine.
Second, there must be a certain minimum surface area of catalyst for the gasses to
come in contact with. This is the reason for the honeycomb design. It provides a
large surface area in a small space. Third, the ratio of exhaust gas to air must be
maintained within very rigid limits [2,3]. These limits are maintained by placing a
special sensor in the exhaust just before the converter. This sensor detects variations
in the ratio and signals the fuel supply system to increase or decrease the amount of
fuel being supplied to the engine.
The dominant factor in automobile activity operational efficiency and profitabil-
ity is maintenance philosophy [11,12]. Engine designers have so far done great
works in ensuring that exhaust gasses are reduced to the minimal, and as much,
harmless, bearing in mind fuel efficiency, engine’s and engine components’ life
[13]. Fuel economy, exhaust emission, and engine noise have become important
parameters not only for engine competitiveness, but also are subjected to legisla-
tion, and it is becoming more severe every few years. Over the years, many have
argued about the actual effect(s) of a leak in the exhaust system on the fuel
consumption rate of an automobile system. This research work seeks to provide
reliable and technical reasons on the point of discourse by developing a model to
evaluate the fuel consumption rate with respect to leakage diameter and length on
the exhaust system.
However, many research efforts have been carried out to measure emission and
fuel consumption rates in the exhaust system and many models have been devel-
oped in this direction. These include:
1.1 Average-Speed Models
Average-speed emission functions for road vehicles are widely applied in regional
and national vehicular operational inventories, but are currently used in a large
proportion of local air pollution prediction models [4,7]. Average-speed models are
based upon the principle that the average emission factor for a certain pollutant and
a given type of vehicle varies according to the average speed during a trip [7]. The
emission factor is measured in grams per vehicle-kilometer (g/km). The continuous
average-speed emission function is fitted to the emission factors measured for
several vehicles over a range of driving cycles (with each cycle representing a
specific type of driving) including stops, starts, accelerations, and decelerations [7].
The measured data for the identified exhaust emission components are plotted,
modeled and then compared to assess their variability.
A number of factors have contributed to the widespread use of average-speed
approach. It is one of the oldest approaches, the models are comparatively easy
to use, and there is a reasonably close correspondence between the required
302 P.K. Oke and B. Kareem
model inputs and the output data made available to the end users. However, there
are a number of limitations associated with average-speed models. These are: (1)
trips having different vehicle operational characteristics and emission levels,
may have the same average speed; (2) all the types of operation associated
with a given average speed cannot be accounted for by the use of a single
emission factor; (3) at higher average speeds, the possible variations in vehicle
operation are limited, while it is greater at low average speed. Besides, the shape
of an average-speed function is not fundamental, but depends on, among other
factors, the types of cycle used in development of the functions [4]. Each cycle
used in the development of the functions represents a given real-world driving
condition, while actual distribution of these driving conditions is not taken into
consideration. Average-speed models do not allow for detailed spatial resolution
in emission predictions, and this is an important drawback in dispersion
modeling.
One of the limitations of average-speed models mentioned earlier was the
inability to account for the ranges of vehicular operation and emission behaviors,
which can be observed for a given average speed; this has made the concept of
“cycle dynamics” useful for emission model developers [11]. The term “vehicular
operation” refers to a wide range of parameters, which describe the way in which a
driver controls a vehicle (average speed, maximum speed, acceleration pattern,
gear-change pattern), as well as the way in which the vehicle responds (engine
speed, engine load).
1.2 Multiple Linear Regression Models (VERSIT
+
Model)
The VERSIT+ model [2] employs a weighted-least-square multiple regression
approach to model emissions, based on tests on a large number of vehicles over
more than 50 different driving cycles. Within the model, each driving cycle used is
characterized by a large number of descriptive parameters (average speed, number
of stops) and their derivatives. For each pollutant and vehicle category a regression
model is fitted to the average emission values over the various driving cycles,
resulting in the determination of the descriptive variables, which are the best
predictors of emissions. A weighting scale is also applied to each emission value,
based on the number of vehicles tested over each cycle and the inter-dependence of
cycle variables. The VERSIT+ model requires a driving pattern as the input, from
which it calculates the same range of descriptive variables and estimates emissions
based on the regression results. As with the other models requiring a driving pattern
as the input, the use of the model is currently restricted to a comparatively small
number of users because of complexity in determining the actual physical variables
involved in it.
25 Effects of Leaked Exhaust System on Fuel... 303
1.3 Instantaneous Models
The aim of instantaneous emission modeling is to map emission measurements
from tests on a chassis dynamometer or an engine test bed in a neutral way [7]. In
theory, the advantages of instantaneous models include the following: emissions
can be calculated for any vehicle operation profile specified by the model user, and
new emission factors can be generated without the need for further testing; and
some instantaneous models, especially the older ones, relate fuel consumption and/
or emissions to vehicle speed and acceleration during a driving cycle. Other models
use some description of the engine power requirement. However, it must be noted
that there are a number of fundamental problems associated with the older genera-
tion of instantaneous models. It is extremely difficult to measure emissions on a
continuous basis with a high degree of precision, and then it is not straightforward
to allocate those emission values to the correct operating conditions [11]. It is stated
in [10] that, during measurement in the laboratory, an emission signal is
dynamically delayed and smoothed, and this makes it difficult to align the
emissions signal with the vehicle operating conditions. Such distortions have not
been fully taken into account in instantaneous models until recently. In order to
apply instantaneous models, detailed and precise measurements of vehicular oper-
ation and location are required, which may be difficult to attain by many public
model users. Consequently, the use of instantaneous models has largely been
restricted to the research community.
1.4 Passenger Car and Heavy-Duty Emission Model
Is a model capable of accurately simulating emission factors for all types of
vehicles over any driving pattern, vehicle load, and gradient. The resulting tool,
passenger car and heavy-duty emission model (PHEM), estimates fuel consumption
and emissions based on the instantaneous engine power demand and engine speed
during a driving pattern specified by the user [7]. The PHEM model thus has the
capability of providing suitable emission resolution for use with micro-simulation
traffic models. The main inputs are a user-defined driving pattern and a file
describing vehicle characteristics. For every second of the driving pattern, PHEM
calculates the actual engine power demand based upon vehicle driving resistances
and transmission losses, and calculates the actual engine speed based upon trans-
mission ratios and a gear-shift model. The engine power and speed are then
used to reference the appropriate emission (and fuel consumption) values from
steady-state engine maps. The emission behavior over transient driving patterns is
then taken into consideration by “transient correction functions,” which adjust the
second-by-second steady-state emission values according to parameters describing
the dynamics of the driving pattern. The outputs from the model are engine power,
engine speed, fuel consumption, and emissions of CO, CO
2
, HC, NO
x
, and PM
every second, as well as average values for the entire driving pattern.
304 P.K. Oke and B. Kareem
1.5 Comprehensive Modal Emission Model
Barth et al. [9] describe the development of a “comprehensive modal emissions
model” (CMEM). The model is capable of predicting second-by-second exhaust
(and engine-out) emissions and fuel consumption, and is comprehensive in the
sense that it is able to predict emissions for a wide range of vehicle and technology
categories, and in various states of condition (properly functioning, deteriorated,
malfunctioning). The main purpose of CMEM is to predict vehicle exhaust
emissions associated with different modes of vehicle operation such as idle, cruise,
acceleration, and deceleration. The model is more detailed than others; it takes into
account engine power, vehicle operation including variable starting conditions
(cold-start, warm start), and off-cycle driving.
CMEM uses a “physical power-demand” modal modeling approach based on a
“parameterized analytical representation of emissions production” [9]. That is the
production of emissions is broken down into components, which correspond to
different physical processes, and each component is then modeled separately using
various parameters which are characteristics of the process. These parameters vary
according to the vehicle type, engine, and emission technology. The majority of the
parameters are known (vehicle mass, engine size, aerodynamic drag coefficient),
but other key parameters are deduced from a test program. Using this type of
modeling approach entails the establishment of models for the different engine
and emission-control technologies in the fleet of vehicles. Once these models have
been established, it is necessary to identify the key parameters in each component
of the models for characterizing vehicle operation and emissions production.
A critical component of the approach is that emission-control malfunction and
deterioration are explicitly modeled. The correct modeling of high-emitting
vehicles is also an important part of the approach. In order to predict emission
rates, the next step is to combine the models with vehicle operating parameters that
are characteristics of real-world driving, including environmental factors (ambient
temperature and air density) as well as dynamic factors (commanded acceleration,
road gradient and the use of auxiliaries (air conditioning, electric loads)) [9].
The complete model is composed of two groups of input: (1) input operating
variables; and (2) model parameters. There are also four operating conditions in the
model: (1) variable soak time start; (2) stoichiometric operation; (3) enrichment;
and (4) “enleanment.” The model determines in which condition the vehicle is
operating at a given moment by comparing the vehicle power demand with thresh-
old values [9].
From the literature, it is clear that the identified emission and fuel consumption
models take into account the various factors affecting emissions and fuel consump-
tion rate in automobile. It is further established that these factors have affected
vehicular emissions and fuel consumption in varying degree. However, efforts on
fuel consumption have since continued but still the effects of a leakage on any part
of the exhaust system in relation with fuel consumption rate have not been
investigated. Sequel to this, effects of leaks on the motor vehicle, specifically on
25 Effects of Leaked Exhaust System on Fuel... 305
fuel consumption rate shall be fully investigated, and a mathematical model shall be
formulated. The rest of the paper is presented as follows: methodology adopted for
the research is present in Sect. 2; results and discussion is in Sect. 3; while Sect. 4
hosts conclusion and future work.
2 Methodology
A vehicle with a new exhaust system (comprising of resonator, catalytic converter,
muffler and its pipes) was used for the research. Before the commencement of the
research, the vehicle was serviced; also, the complete exhaust system of the vehicle
was replaced with the new one to ensure a good condition and a reliable result
during the experiment. During the period of the experiment, the vehicle was
stationary and the engine allowed operating in slow running mode in order to
eliminate the effects of speed, acceleration, drive pattern, and cruise control on
the fuel consumption rate. The following items were used during the course of the
experiment; two hoses (with diameter 1.5 cm), two tightening rings, a measuring
can, veneer caliper, a stop watch, and 50 l of fuel. The hose from the fuel tank was
connected to the external measuring can with the 1.5 cm diameter hose with
two tightening ring clip. The external fuel tank was filled with 1 l of fuel before
the commencement of the experiment. Four tests were conducted with two trials
per test on the exhaust pipe at two different times: first, the effect of varying the
diameter of the leak on fuel consumption rate; second, the effect of varying
the length of exhaust system at which the leak occurs on fuel consumption rate.
The following describe the various steps taken while carrying out the experiment
(the vehicle used is a Honda CRV jeep).
Step 1: The vehicle was made to stay in a fixed position. The default exhaust pipe of
the vehicle was removed and replaced with a new one.
Step 2: The pipe leading to the fuel tank was disconnected after the fuel pump. The
pipe from the fuel pump was then extended with the first hose, using one of the
tightening rings.
Step 3: The fuel return pipe was disconnected from the injector. The second
hose was then connected to the injector at the fuel return outlet, using the other
tightening ring.
Step 4: The measuring can was used as the external tank. A little quantity of fuel
was poured into the external tank and with the intake and return hoses put into the
external tank; the engine was allowed to operate under slow running for 45 s. This
was done so that the intake and return hoses will retain some amount fuel in order
that our results may not be affected.
Step 5: The overall length of the complete exhaust system was measured, and its
value was recorded as L
0.
306 P.K. Oke and B. Kareem
Step 6: The measuring can was filled with 1 l of fuel and the engine was made to
start under slow running without any puncture on the exhaust pipe; the time taken
to use up the 1 l of fuel was taken (using a stop watch) and its value recorded as t
0
.
Step 7: Step 6 was repeated for leak diameters 5, 10, 15, and 20 mm on the
following locations of the exhaust system: between the exhaust manifold and
catalytic converter (43.7 cm from the exhaust manifold outlet); between the cata-
lytic converter and silencer (138.40 cm from exhaust manifold); very close to the
silencer outlet (233.70 cm from exhaust manifold), and muffler mouth (355.60 cm
from exhaust manifold)
Step 8: The leaks were repaired after every puncture for each location on the
exhaust pipe using oxy-acetylene gas welding process.
Step 9: Steps 6–8 were carried out again and their average values were used.
During the course of this experiment, it was ensured that: the vehicle used was
serviced shortly before the commencement of the experiment; the correct quantity of
fuel was used and the accurate time taken; the engine was made to run at a constant
speed; and the leaks were properly repaired after each exercise. The configuration of
the exhaust system used for the experiment is shown in Fig. 25.1.
3 Results and Discussion
The results obtained from four trials experimentation on exhaust system of overall
length, L
0
of 419.10 cm are shown by Fig. 25.2. A leak/hole in an automobile
exhaust system affects not only the health of its driver and passengers but also the
fuel consumption rate and the engine performance. From the experiment and
the results obtained from the experiment, the following deductions can be made:
(1) the rate at which fuel is consumed increases with the hole diameter, regardless
of its location on the exhaust system. That is, the rate of fuel consumption varies
directly (although this might not be a proportionate variance) with the hole diame-
ter; (2) considering the entire exhaust assembly, the region of the catalytic converter
Fig. 25.1 A complete
exhaust system
25 Effects of Leaked Exhaust System on Fuel... 307
consumes fuel the most. The catalytic converter is responsible for the increased fuel
consumption rate at point “2.” This is because it uses oxidation catalyst made up of
ceramic beads coated with platinum to reduce HC and CO emissions. Due to
catalytic action, the converter takes more fuel (for burning to achieve the desired
essence); to convert HC, CO, and other pollutants to water vapor and CO
2
; (3)
the rate of fuel consumption decreases with the length of the leak from the exhaust
manifold; and (4) as the length and hole approaches the muffler, noise level reduces.
Since this research considers two major parameters-diameter and length of
leak-hole with respect to a leaking exhaust system, the general effects of hole
(diameter) and length variations are as follows: (1) an increase in back pressure-
design factor designers has been battling with for years—trying to reduce it to its
barest minimum; (2) for vehicles using fuel injectors, the leak alters or interferes
with the oxygen sensor reading. Thereby sending a wrong reading to the ACS
Fig. 25.2 Fuel consumption rate with varying leak location on the manifold
308 P.K. Oke and B. Kareem
(automatic control system)—this results in inefficient combustion, poor fuelling
and poor power; (3) enhances catalytic converter damage—which is quite expen-
sive to replace; (4) in a more severe case, may cause backfire; and (5) more fuel
is consumed. The issue of length or location is also paramount. For instance, if
the hole is before the sensor, it will affect the reading. Also if it is before the
catalytic converter, it could damage it, else no effect; if it is on the muffler-noisier
exhausts results.
The summary of the results of the four trials is shown in Table 25.1.
The outcomes are modeled using multiple linear regression analysis (25.1) with
the following parameters [12–14]:
a¼the diameter of the hole/leak on the exhaust system
c¼the location (length from the exhaust manifold outlet) where the hole/leak
occurs
l¼the fuel consumption rate
b¼coefficient of entity
i¼the ith terms in each trial
n¼number of terms being considered
nb0þb1X
n
i¼1
aiþb2X
n
i¼1
ci¼X
n
i¼1
li(25.1)
The linear multiple regression analysis results for the first, second, third, and
fourth trials are, respectively modeled as,
l1¼2:190000 þ0:084000a0:00000282c(25.2)
l2¼0:920000 þ0:0894000aþ0:00000578c(25.3)
l3¼3:510000 þ0:128400a0:00000414c(25.4)
l4¼2:630000 þ0:135000aþ0:00000811c(25.5)
From the models, fuel consumption rates can normally be predicted (R20.9)
with a given leakage diameter and location on the exhaust system.
Table 25.1 Experimental results for modeling
Diameter of leak, a, mm 5.00 10.00 15.00 20.00
Fuel consumption rate, l
1
, where leak location , cis 43.7 cm 2.60 3.20 3.46 3.89
Fuel consumption rate, l
2
, where leak location , cis 138.4 cm 1.59 1.69 2.04 2.78
Fuel consumption rate, l
3
, where leak location , cis 233.7 cm 4.15 4.86 5.43 6.10
Fuel consumption rate, l
4
, where leak location , cis 355.60 cm 3.31 3.92 4.64 5.29
25 Effects of Leaked Exhaust System on Fuel... 309
4 Conclusion and Future Work
From the research work carried out, it has been established that the rate of fuel
consumption increases, approximately linearly, with increase in diameter of hole on
the exhaust system; also that the location of leakage has a somewhat negligible
effect on fuel consumption rate. Therefore, leakage location can only be considered
for design purposes to maintain design accuracy. The knowledge of the effect of a
leaking exhaust system on the rate of fuel consumption will help car owners and
drivers to make better choices of maintenance policies for their exhaust system.
The modeling results would help the vehicle owners in monitoring and controlling
their fuel expenses. Vehicle owners could say, the hole is here or there, or on this or
that, so it can still be tolerated. The models would also help vehicle designers to
look into some design considerations at such points where the effect of leak is
disastrous. This will promote better fuel economy and cleaner exhaust output.
Having known the size of leaks and the locations, the rate at which fuel is consumed
can be quantified. The study may be extended to other exhaust systems of automo-
bile to verify the efficacy of the outcomes of this research.
Acknowledgment The authors thank the management of the Federal University of Technology,
Akure, Nigeria for providing an enabling environment for carrying out this study.
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