Analysis of the losses associated with the Valve Train System
and External Pumps of an Automotive Engine
Mechanical Engineering Project Proposal
Hrant Khatchadourian
Rensselaer Polytechnic Institute
Department of Engineering
Fall 2011 Semester
10/13/11
Abstract:
This study presents the analysis of potential horsepower (power) gains associated with
the replacement of a mechanically linked valve train system and other necessary external
components/pumps used in an Otto cycle or (car engine) with one that would be electronically
power and controlled. A refined overhead cam system will be analyzed in order to simplify
equations used to solve for frictional and inertial losses from the mechanically linked
components in the valve train. Only the necessary external pumps such as, the water pump, airconditioner pump and power steering pumps will be analyzed. The external pumps are assumed
to be belt driven directly off the crankshaft as seen in a typical car engine setup. While removing
these parts altogether will obviously free up power delivered from the engine, an analysis will be
made to determine the power requirements of electronically controlled and operated components
used to replace the functions of the mechanically operated parts that would be removed. The
power freed up and the power required by the replacement parts will be compared with each
other to determine an overall potential horsepower gain.
2
Table of Contents:
Abstract……………………………………………………………………………………………2
Table of Contents………………………………………………………………………………….3
Introduction…………….………………………………………………………………………….4
Problem Description........................................................................................................................8
Methodology………….………………………………………………………………………….11
Required Resources……………………………………………………………………………...14
Expected Outcomes……………………………………………………………………………...15
Deadlines…………………………………………………………………………………………16
References………………………………………………………………………………………..17
3
Introduction:
In this report a reciprocating piston-cylinder car engine will be analyzed since most car
engines are of this type. The most common reciprocating piston-cylinder car engine is a four
stroke engine. Figure 1 shows a single cylinder and the basic parts to make it function properly,
where “V” is for valve, “I” is for intake (pointing at the camshaft), “E” is for exhaust (pointing at
the camshaft), “S” is for sparkplug, “P” is for piston, “R” is for connecting-rod, “C” is for
crankshaft and “W” is for water-jacket or coolant.
Figure 1 – Reciprocating Piston-Cylinder Engine (4)
The term four-stroke engine comes from the fact that the piston travels a full stroke in the
cylinder four times for every two revolutions of the crankshaft. The 4 strokes that complete a
full cycle are the intake stroke, compression stroke, combustion (power) stroke and the exhaust
stroke. During the intake stroke, the intake valves open in order to draw in a fresh charge of a
combustible mixture of fuel and air as the piston travels down the cylinder. During the
compression stroke, both intake and exhaust valves close in order for the piston to compress the
4
charge of gas mixture on its way back to the top of the cylinder. This stroke requires a work
input from the piston to the cylinder contents in order to compress it. When the piston reaches
top dead center (TDC) in the cylinder, the spark plug ignites the gas mixture into combustion
causing it to expand. Work is done by the expanding gas onto the piston as it travels back down
the cylinder to bottom dead center (BDC). Finally, during the exhaust stroke, the exhaust valves
open to discharge the combustion by-product out of the cylinder as the piston travels to TDC
again.
The cycle of a reciprocating piston-cylinder car engine can be analyzed as an ideal airstandard Otto cycle in thermodynamics represented by the “pv” (pressure vs. volume) and “T-s”
(temperature vs. entropy) diagrams shown in Figures 2 and 3 respectively. The 4 processes in an
Otto cycle are setup to analyze the 4 strokes of an engine, assuming that the heat addition
(ignition of the gas mixture) occurs instantaneously while the piston is at TDC and that the
engine has already drawn in a fresh charge of air in which no input work is required, as shown in
the blue and green lines in Figure 2. Although these 2 processes are critical to the operation of a
real engine, these assumptions are made to simplify the analysis of the thermodynamic Otto
cycle.
Figure 3 – “p-v” Diagram of an Otto Cycle (5)
5
Figure 3 – “T-s” Diagram of an Otto Cycle (5)
The process from 1 to 2 is an isentropic compression of the air as the piston moves from
BTD to TDC. This process represents the compression stroke of an engine and requires a work
input in order to compress the air. Process 2 to 3 is a constant-volume heat transfer to the air
from an external source while the piston is at TDC. As said before, the intent of this process is to
represent the ignition of the gas/air mixture and the subsequent rapid burning. Process 3 to 4 is
an isentropic expansion which models the power stroke of an engine and produces a work output.
Finally, process 4 to 1 completes the cycle by a constant-volume process in which heat is
rejected from the air while the piston is at BTC.
This thermodynamic cycle will not operate unless the cylinders have a fresh charge of air
and fuel to combust as well as have a way to exhaust used CO2 after combustion. The purpose
of a valve train system in a car engine is to open/close the intake/exhaust valves to make this
process work. Since the engine is rotating at several thousand revolutions per minute (RPM),
this process is constantly occurring. In order for the valves to know when to open and close, the
valve train is directly coupled off the crankshaft through a timing chain or belt attached to cam
gear. The cam gears, which rotate at half the crankshaft speed, are attached to camshafts which
open and close engine valves at the precise time they are needed. All of these mechanically
linked parts rotate, roll and move very fast as engine speeds are increased. Although lubricated,
all of these moving parts cause heat due to friction which can be calculated as a loss. Also, with
all of the added mass in the valve train system, moving parts cause loading on the engine in
which an inertial loss can be calculated.
6
There are other external pumps used in a typical car engine that absorb power. The
average efficiency of a car engine or Otto-cycle engine is about 25-35% (www.wikipedia.com).
Most of the loss in a car engine is in the form of waste heat. As thousands of mini controlled
explosions take place in each cylinder to create power, the entire engine block gets hot from this
waste heat. In order to keep the engine block and cylinder walls at a normal operating
temperature where material degradation and fatigue does not occur, coolant must be pumped
throughout the engine. Although called a water pump, a mixture of antifreeze (glycol) and water
is pumped by the water pump into the engine and cooled by forced convection through a heat
exchanger (radiator) mounted in the front of the car. This pump as well as the power steering
pump and air-conditioner pump (compressor), operate by a direct coupling with the crankshaft
pulley via belts. All these pumps generate added loss to the overall output of the cars engine.
7
Problem Definition:
Engine efficiency can be define by the relationship between the total energy contained in
the fuel, and the amount of energy used to perform useful work. Today, in a time where oil
prices and demands are extremely high, it is necessary to utilize every last bit of power out of an
engine. In the United States and other countries there are laws and restrictions on how low an
efficiency car manufacturers can have in their new line of cars. Oil companies do their best to
keep gasoline engines on the ground, and hybrid vehicles still have a learning curve and lack of
high performance appealing to consumers. An average efficiency of 30% out of gasoline
engines is very low from an engineers’ standpoint, and this is the reason why methods of getting
more efficiency in reciprocating piston-cylinder gasoline engines (more work output for the same
amount of heat input) is essential in today’s world.
The 2 main components of an engine block that carry out work are the pistons and
crankshaft, as shown in Figure 4. All other accompanying components (such as valves, valve
springs, camshafts, cam gears, water pump, power steering pump, etc.) add load to the work
being done by the engine. The objective of this report is to analyze valve train system and
external pumps power losses that can be freed up to increase the amount of output work done by
the engine. Losses due to friction and moment of inertia will be analyzed for the valve train and
losses to do pump performance and belt tension will be analyzed for the main external pumps.
Figure 4 – Basic Engine Internal Diagram (6)
8
Figure 5 shows a flow diagram of how power enters and leaves the engine to perform
different tasks. In a typical engine setup like the one being analyzed in this report, it can be seen
in Figure 5 that the blocks in red are being operated mechanically by the engine to operate the
valve train system, water pump, air conditioning pump, alternator and power steering pump. In
the case for the valve train system a chain is directly coupled to the engines crankshaft to rotate
the cam gears which in turn rotate the camshafts in order to operate each valve to let fresh air
into the cylinder or let used fuel gases out of the cylinder. If these multiple moving parts could
be replaced by a single electric operated part, then used power would be freed up and could be
utilized by the transmission to move the car.
Valve
Chain
Air
Cam Gear
Camshaft
Engine
Fuel
Water
pump
A/C Pump
Output
Power
Chain
Alternator
Power
Steering Pump
Figure 5 – Flow Diagram of Typical Engine Power Distribution
Figure 6 shows a flow diagram of how the power would be distributed in the alternative
car engine design that this report seeks out. The blue blocks represent the electric operated parts
that replace the mechanically operated parts in red. It can be seen in the Figure 6 flow diagram
that there are far less mechanically operated moving parts. This would not only free up power,
but would make each part modularly separate from being linked to the engines crankshaft,
making easier to take apart or replace. Also, these parts would not be operated unless required
9
by the user, unlike in typical engine setups where these parts are always linked to the engines
crankshaft.
Valve
Solenoid
Air
Water
pump
Engine
Chain
Alternator
A/C Pump
Fuel
Power Steering
Pump
Output
Power
Figure 6 – Flow Diagram of Alternative Engine Power Distribution
10
Methodology:
Since there are too many different variations of engines to analyze such as, the number of
cylinders (inline 4 or inline 6, straight 4 or straight 6, boxer, V-6, V-8, V-10, V-12) with each
possibly having different valve train setups (such as overhead camshaft, push-rod systems, direct
acting camshaft), a simplified direct acting overhead camshaft setup in a boxer 4 cylinder engine
is taken as a model and will be analyzed (much like a 2006 Subaru WRX STI engine), as shown
in Figure 7. The 4 cylinder boxer engine has 2 cylinders on each bank horizontally opposing
each other. Each bank will need its own intake and exhaust camshaft to operate the valves and
each cylinder will have dual intake and exhaust valves. Therefore in this particular engine setup
each camshaft will operate 4 valves, which collectively totals 4 cam gears, 4 camshafts, 16
valves and 16 valve springs, as shown in Figures 8 and 9 respectively.
Camshaft open durations will need to be analyzed to determine how long the valve spring
is open and closed during one revolution. Valve spring spring-constants (k), seat load and open
pressure loads are given by manufacturers. These values can be used with the camshaft open
duration values to determine a continuously applied normal spring force directed from the spring
onto the camshaft cam lobe. From the normal spring force, the friction force of the tappet onto
the cam lobe can be determined, incorporating a friction coefficient of common slide bearings or
journal bearings. Therefore, the torque required to move the camshafts can be calculated and in
turn the required power can be determined depending on angular velocity and acceleration.
Besides calculating the friction force associated with the valve train, the moment of
inertia generated by the valve train will also be calculated. A good estimation can be obtained on
the total mass of all the valve train components and their relative angular velocities will be
calculated in order to determine the moment (torque) required to move those parts. Both the
friction force translated into a torque and the moment of inertia can be added together to
determine an overall torque required to overcome parasitic losses in moving the valve train.
11
Figure 7 – Front View of 06 Subaru STI Engine/Valve Train (7)
Figure 8 – 06 Subaru STI Camshafts (8)
12
Figure 9 – 06 Subaru STI Valve Springs (9)
13
Required Resources:
Since this project is not one that is normally calculated, a variety of different courses will
be used in order to solve for different loss parameters. A spring-mass analysis will need to be
done using dynamics. These forces will translate into a loading on a camshaft in which
mechanics of materials textbook as well as dynamics can be used to determine a rotational
torque. In conjunction to acquiring that torque, fluid mechanics will be used to account for a
friction between two surfaces separated by a hydrodynamic fluid. Friction coefficients will need
to be acquired from outside (online) sources that have tested various slide and journal bearings.
Moment of inertia calculations will also be done using a combination of mechanics of materials,
dynamics and physics. Overall engine operation and performance will be analyzed by
thermodynamics.
For sanity check to make sure calculated values are within reason, different online
sources will be used to acquire values of case studies they have done to see what values of valve
train friction and torques were measured. Even though there are too many parameters in engine
design to accurately compare any calculation to a measured value, it will relatively give good
approximations of the amount of power required to operate the valve train system in an engine.
Computer access with Microsoft Excel and Microsoft Word will be required to do calculations
and analysis.
Computer access with Microsoft Word and Microsoft PowerPoint will be required to do all
written and presentation aspects of this project.
Other Microsoft Office programs may be used for visuals.
14
Expected Outcomes:
The expected outcome from the analysis of this report would be in the range of an
additional 30 to 50 HP. Obviously these losses would vary depending on the engine
configuration and RPM, but an average engine design is being analyzed at different operating
RPMs that would normally be seen in a typical engine. Considering all the bearings, couplings
and forces acting specifically on the valve train, it can be assumed that more than half (most
likely around 65-75%) of the engine losses analyzed in this report would be attributed to valve
train operation.
15
Milestones:
Milestones
Submit Proposal and Brief Presentation
Refine exactly what losses will need to be analyzed
- Friction losses on cam lobes
- Friction losses in slide/journal bearings
- Torque requirements of external pumps
Form equations that will be needed for analysis
First Progress Report
Refine equations to account for more realistic conditions
Make final assumptions to clarify work and relate to expected values
Second Progress Report
Determine power requirements of electronic replacement parts
Final Draft
Final Report
Completion Date
9/30/2011
10/7/2011
10/14/2011
10/21/2011
10/28/2011
11/4/2011
11/11/2011
11/18/2011
12/2/2011
12/16/2011
16
References:
(1) R.C. Hibbeler, Engineering Mechanics, Dynamics 10th Ed, Pearson Prentice Hall 2004
(2) Michael J. Moran, Howard N. Shapiro, Fundamentals of Engineering Thermodynamics 6th
Ed, John Wiley & Sons, Inc. 2008
(3) Mechanics of Materials book
(4) Wikipedia, http://en.wikipedia.org/wiki/File:Four_stroke_engine_diagram.jpg
(5) Wikipedia, http://en.wikipedia.org/wiki/Otto_cycle
(6) Wikipedia, http://en.wikipedia.org/wiki/Valvetrain
(7) Online photo from website, http://image.superstreetonline.com/f/17464591+w750+st0/130
0904_086_z+tokyo auto_salon+subaru_boxer
(8) Online photo, http://www.jscspeed.com/images/catalog/category1990_thumb.jpg
(9) Online photo, http://www.jscspeed.com/images/catalog/category2084_thumb_mid.jpg
17