ME 444
Assignment 7 Due Date: Oct. 17th 2013
1. Hand sketch a turbocharger engine configuration and label the major parts.
2. If a 2.6 L engine can produce 100 kW, calculate how much power it could produce when the
intake system can be boosted by 0.5 bar. State all the assumptions required to make this
calculation given that the volumetric efficiency doesn’t change significantly.
In this formula, swept volume, engine speed, fuel combustion efficiency, heating value of fuel,
and volumetric efficiency would be the same for both engines. The remaining parameters are
the air density and air-to-fuel ratio.
Air-to-fuel ratio:
a) Assume that the normally aspirated gasoline engine was running slightly rich (recommended
in practice to avoid high burned gas temperatures).
Hence, assume an AFR of 12.5 for the NA engine.
b) In contrast for boosted engine, the AFR should be maintained at the peak value of allowable
range i.e rich mixture. This is employed in order to avoid knocking probability due to increased
temperatures of burned gases had the mixture was lean. This latter case of lean mixture
knocking is because of the increased air density by boosting.
Hence, assume an AFR of 13 for the turbo engine.
Air density:
From the familiar ideal gas equation, pressure S = 𝜌𝑅𝑇/𝑀.
In this equation, R and M remain same for both engine cases. Temperature T would be the same
if we had an air charge cooler.
a) Hence, assume that an air charge cooler is included in the turbo configuration.
b) Assume that the intake pressure of the normally aspirated engine is 0.7 bar.
Consider subscripts 1 and 2 to represent normally-aspirated and boosted engine parameters,
respectively. Then we have,
P1/P2 = (S1*AFR2)/(S2*AFR1)
100/P2 = (0.7*13)/[(0.7+0.5)*12.5] → P2 = 164.83 kW
**If a student considered the AFRs to remain same, points would still be given as
long as intake pressure ‘S1’ was taken a value less than 1 bar.
3. Explain how gasoline direct injection (GDI) can be used to reduce turbo lag. What special
hardware on an engine is required to make sure that this re-configuration will work? If any,
mention the disadvantages to the proposed system.
Gasoline direct injection allows the engine to take more air than needed so as to burn extra fuel
sprayed at the exhaust side. This process known as “Scavenging” reduces the turbo lag. In
addition, the GDI system offers more control over the fuel flow for each cylinder. This
reconfiguration with turbo requires a WRAF sensor to accurately control the AF ratio.
The disadvantage of the GDI is the usage of expensive fuel injectors and high-pressure fuel
pumps. Because the injector tips are mounted right into the combustion chamber, the materials
in the injector have to be very good quality and that costs money. Also, the high pressure
needed to inject fuel directly into the cylinders means that more expensive high-pressure fuel
pumps are required. In addition, these pumps are mechanically driven from the engine which
adds to the complexity.
4. If a set of fuel injectors for a particular engine have different flow characteristics, the
cylinders may show unequal performances. Make a sketch of how can we adjust the fuel flow
for each injector (using modern controls) to attain maximum performance? Your answers
should not exceed one-half (1/2) page.
5. Why would it be important to know the barometric pressure when developing an engine
control system. How can one estimate this pressure in a cost-efficient manner and what are the
potential problems in doing so? Additionally, how can one tackle this problem for a car running
on Pikes Peak (Rocky Mountains, Colorado)?
Knowledge of barometric pressure for the engine computer is required because the ambient
pressure is directly linked to the breathing capacity of the engine cylinders. A higher outside
pressure would make the engine job easier in terms of intake air flow. However, too much of
outside pressure would make the engine harder to let out the burned gases during exhaust.
Hence, a barometric pressure sensor is desired to be installed in the engine control system but is
generally expensive. Alternatively, the barometric pressure can be estimated from the available
measured variables. For example,
Barometric pressure = Intake manifold pressure + delta_P (evaluated based on operating
conditions).
Based on an initial testing, it was found that the calibrated value for delta_P was not a constant
and was varying for different altitudes yet for same flow.
This problem with the abovementioned algorithm can be dealt with, especially for higher
altitude environments such as Pikes Peak, by implementing a “compressible flow model.” In this
algorithm, pressure ratio (PManifold/Patm) is considered instead of delta_P as seen earlier.
6. What limits the operating of a diesel engine at higher speeds which would otherwise result in
high power output?
Diesel engines, unlike their gasoline counterparts, are built sturdily to withstand extremely high
pressures seen during compression and power strokes (150 bar vs. about 60 bar for a gasoline
equivalent). So, the diesel engine is made-up of robust components making it heavier on the
whole. Such a heavy engine carries high inertial forces which would become worse if tried to run
at high speeds (recall engine balancing issues due to reciprocating and rotating masses). Hence,
the power density of diesel engines is speed limited.
7. Hand sketch a supercharger system and compare it with a turbocharged system by discussing
the advantages and disadvantages of each.
Recalling the turbocharger system sketched earlier, it can be seen that the major difference
between a supercharger and turbocharger system is that in the former, the engine crankshaft
turns the compressor while in the latter, the exhaust gas rotated turbine drives the compressor.
On further comparison between the two, following interpretations can be made:
1+) The mechanically supercharged system gets rid of turbine and corresponding wastegate
requirements as seen in turbocharged system.
1-) The compressor of a supercharged system takes about 15% of the engine power which is a
significant amount.
2+) A turbocharged system makes use of the exhaust energy (about 30-40% of fuel energy)
which would otherwise go wasted.
2-) The pumping losses are increased due to the requirement of high exhaust gas temperatures.
8. How does a turbocharger’s wastegate work and what is its purpose?
A Wastegate is simply a turbine bypass valve. It works by diverting some portion of the exhaust
gas around, instead of through, the turbine. This limits the amount of power that the turbine
can deliver to the compressor, thereby limiting the turbo speed and boost level that the
compressor provides. It is usually controlled by a pressure actuator that is connected to
manifold pressure. When preset pressure limits are exceeded, the actuator progressively opens
the wastegate; allowing exhaust flow to bypass the turbine, thus regulating manifold boost
pressure.
9. On a P-V diagram for a 4 stroke cycle turbocharged engine, show the energy that is available
to drive the turbine of a constant pressure and pulse turbocharger, respectively.
The non-shaded area with slant lines is the energy available to drive the turbine of a constant
pressure turbocharger.
The shaded area with slant lines is the energy available to drive the turbine of a pulse
turbocharger.
10. Why are the wave dynamics of an engine important for turbocharging?
Knowledge of wave dynamics generally determined through engine analysis tools such as WAVE
and GTPOWER is important for the turbocharged engine design. Pressure waves are generated
during the intake flow motion and during “blowdown” process of exhaust gases out of the
cylinder. These waves propagate along the intake and exhaust manifolds till they reach the
intake and exhaust ports and reflect back from there. These waves change the levels of pressure
gradient and would eventually block the gas flow in the exhaust manifold if the exhaust valve
was not properly actuated. Hence, the valve timings are affected due to the back pressures and
in turn these would affect the amount of exhaust available to rotate the turbo’s turbine.
11. In a compressor map as shown here, explain what happens at a) Surge line, b) Choke line,
and c) Maximum speed line:
a) Surge line is the left hand boundary of the compressor map. Operation to the left of this line
represents a region of flow instability. With too small a volume flow and too high a pressure
ratio, the flow can no longer adhere to the suction side of the blades, with the result that the
discharge process is interrupted. The air flow through the compressor is reversed until a stable
pressure ratio with positive volume flow rate is reached where the pressure builds up again and
the cycle repeats. This flow instability continues at a fixed frequency and the resultant noise is
known as "surging." Continued operation within this region can lead to premature turbo failure
due to heavy thrust loading.
b) Choke Line is the right hand boundary of the compressor map. The compressor efficiency
drops rapidly past this point. The maximum centrifugal compressor volume flow rate is normally
limited by the cross-section at the compressor inlet.
c) The maximum speed line signifies that any operation above this line would induce large
amounts of stresses and vibrations eventually leading to the compressor failure.
12. How does the turbocharger permit fuel economy improvement via downsizing?
As seen earlier, power density of an engine is a function of three parameters: Swept volume,
engine speed, and air density. Power density can be improved by increasing the engine speed
but is an expensive choice for SI engines and physically limited for diesel engines. The remaining
options are swept volume and air density.
Swept volume could be increased by considering a larger engine but results in the following
issues: a) A larger engine involves larger moving components and hence, high friction and
inertial forces. These lead to high vibrations and balancing issues. b) A larger engine affects the
packaging task on the vehicle chassis. c) A larger cylinder involves larger valves that would
consume lot of energy during their actuations.
Based on these issues, it is usually not recommended to increase the swept volume by choosing
a larger engine cylinder. Turbocharged engine improves the power density and fuel economy by
increasing the third parameter i.e. air density. By increasing the intake air density, a downsized
or smaller turbo engine is capable to provide the power equivalent to that of a larger naturally
aspirated engine. Further, a smaller engine would get rid of all those issues as seen for a larger
engine, thereby, optimally uses the fuel energy from combustion.
13. Explain the following technical challenges related to turbocharging an IC engine and provide
at least one potential solution for each: a) Temperature limits, b) Transient response, c) Energy
recovery, and d) Turbine sizing.
Temperature limits: A higher exhaust temperature would provide higher output from the
turbine. However, presence of such higher temperatures e.g. 1050 oC would affect the material
surfaces of the turbine housing and wheel.
Probable solution: Employing heat-resistant cast steel material for turbine housing and nickelbased superalloys for the turbine wheels.
Transient response: Turbo lag. The exhaust gas possess the desired thermodynamic properties
(temperature and pressure) to run the turbine only above certain engine RPM. So, turbo lag is
evident during engine idling and low load conditions.
Probable solution: Variable geometry turbine and/or compressor.
Energy recovery: The EGR system assists in controlling the NOX production in engines by
reducing the in-cylinder temperatures. However, feeding this recirculated gas into the turbo
inlet and mixed with the fresh charge air would increase the inlet temperature well above the
ambient. The increase in inlet temperature from EGR, combined with the corrosive and abrasive
effects of the exhaust gas, pose an increased challenge to the tensile and fatigue strength of
compressor wheel.
Possible solution: Employing titanium compressor wheel made from both CNC-billet and
investment-castings.
Turbine sizing: While smaller turbine wheel spins the compressor wheel faster, it may block the
exhaust gas flow causing buildup between the combustion chamber and the turbine. This
blocking characteristic can be represented by the exhaust backpressure.
Possible solution: Consider a turbine having a wheel diameter within 15% to that of the
compressor.
14. Explain briefly about regulated two-stage turbocharging and the actuators used in this
system.
The regulated 2-stage turbocharger consists of two turbochargers of different sizes connected in
series that utilize bypass regulation. The exhaust mass flow coming from the cylinder flows into
the exhaust manifold first. Here it is possible to expand the entire exhaust mass flow using the
high pressure turbine (HP) or to redirect some of the mass flow through a bypass to the low
pressure turbine (LP). The entire exhaust mass flow is then utilized again by the low pressure
turbine (LP). The actuators used in the BorgWarner R2STM system are: 1) a HP turbine bypass
valve that splits up the mass flow between HP and LP stage. 2) Compressor bypass valve that
bypasses the HP-compressor. 3) A LP turbine wastegate valve that controls mass flow through
the LP turbine.
15. (a) What happens if the back pressure on the turbine is increased? What happens in the
engine under these conditions?
Excessive back pressures at the turbine could create mechanical defects such as blade surface
wear due to cavitation or similar effects. Also, these pressures can increase the likelihood of
failure of turbocharger seals, resulting in oil (turbine lubricant) leakage into the exhaust system.
At increased back pressure levels, the engine has to compress the exhaust gases to a higher
pressure which involves additional mechanical work. This can lead to an increase in fuel
consumption, PM and CO emissions and exhaust temperature (and hence possible NOX).
(b) Why is an intercooler recommended for a turbo engine?
An intercooler reduces the temperature of the compressor outlet gas before it is fed to the
engine cylinder. A decrease in intake air charge temperature sustains use of a denser intake
charge into the engine which would provide relatively thorough combustion. The lowering of the
intake charge air temperature also eliminates the danger of pre-detonation (knock) of the
fuel/air charge prior to the spark ignition.