Now that we have introduced knock/detonation, contributing factors and ways to decrease the likelihood of detonation, let’s talk about compression ratio.
Compression ratio is defined as:
where CR = compression ratio Vd = displacement volume Vcv = clearance volume
The compression ratio from the factory will be different for naturally aspirated engines and boosted engines. For example, a stock Honda S2000 has a compression ratio of 11.1:1, whereas a turbocharged Subaru Impreza WRX has a compression ratio of 8.0:1. There are numerous factors that affect the maximum allowable compression ratio. There is no single correct answer for every application. Generally, compression ratio should be set as high as feasible without encountering detonation at the maximum load condition. Compression ratio that is too low will result in an engine that is a bit sluggish in off-boost operation. However, if it is too high this can lead to serious knock-related engine problems. Factors that influence the compression ratio include: fuel anti-knock properties (octane rating), boost pressure, intake air temperature, combustion chamber design, ignition timing, valve events, and exhaust backpressure. Many modern normally-aspirated engines have well-designed combustion chambers that, with appropriate tuning, will allow modest boost levels with no change to compression ratio. For higher power targets with more boost , compression ratio should be adjusted to compensate. There are a handful of ways to reduce compression ratio, some better than others. Least desirable is adding a spacer between the block and the head. These spacers reduce the amount a “quench” designed into an engine’s combustion chambers, and can alter cam timing as well. Spacers are, however, relatively simple and inexpensive. A better option, if more expensive and time-consuming to install, is to use lower-compression pistons. These will have no adverse effects on cam timing or the head’s ability to seal, and allow proper quench regions in the combustion chambers.
There are three major reasons for turbocharger failure:
and a lack of regularly scheduled maintenance.
Lubrication Failure –
Proper cooling and lubrication are vital to turbocharger operation.
Turbochargers are driven by hot exhaust gases
exiting the combustion chamber and, therefore,
are subject to extreme temperatures. In a Burst
& Containment Document authored by Honeywell , their Garrett Engineering group
provides an operating description of its smallest
turbo product. The article states that the
turbocharger’s impellers operate up to 200,000
rpm and the exhaust gas temperature can reach a
max of 1800°F, causing the turbine housing to
glow red under certain driving conditions.
Circulating engine oil through the turbocharger
to remove heat and provide lubrication to
internal components ensures functional
operation. A lack of lubrication from degraded
engine oil or insufficient delivery from an
obstructed oil line can cause increased friction
and temperatures. In extreme cases of poor
lubrication and high operating temperatures,
moving components can become seized, locking
up the turbocharger and disabling operation of
the vehicle. Excessive Condensation –
Many high performance turbochargers are known for their
variable geometry turbines (VGT or VNT).
These units have controlled vanes, which help
increase or decrease impeller speed within the
turbine’s housing. Seized or sticking vanes
cause performance failures on this type of turbo.
Rust formations on the vanes, created by
excessive condensation build-up when a vehicle
is idle for extended periods of time, is the cause
of failure. Condensation can enter the tail pipe
or an exhaust manifold gasket leak. Typically,
this occurs from irregular or sporadic driving.
Regular on-the-road vehicle use will help burn
condensation and prevent rusting.
Lack of Maintenance –
Turbocharged engines require thorough care with stable, regularly
scheduled maintenance cycles for lubrication
systems, air filters, seals and gaskets. As
previously noted, catastrophic failure can occur
when engine oil changes become infrequent and
fail to meet recommended scheduling intervals.
Air filter maintenance is vital to eliminate
foreign object damage (FOD) to the charged air
impeller. Research shows that under normal
driving conditions, most medium-duty diesel
vehicles have an average operation life of 10
years or 100,000 kilometers on their exhaust systems.
At 100,000 kilometers, inspecting and cleaning the
intercooler system and removing any cause of
FOD is highly recommended. After excessive
miles, turbochargers sometimes experience
pressure loss caused by worn seals (leakage)
and/or worn impeller (leakage). Seals will easily
damage under extensive pressure cycles,
especially sealing rings for rotating parts.
Foreign object damage to impeller, as well as
chipped or bent vanes on impellers, will reduce
required energy efficiency to increase
combustion pressure (power).
We repair and sell all brands of turbochargers. Our vast knowledge assures you of the best quality and assembly available. All our repaired turbochargers are test run and checked for oil leaks prior to delivery, guaranteeing peace of mind.
Upon Receipt of damaged turbocharger and assessment is done to establish cause of failure and advising client of possible engine damage. The turbocharger is stripped and examined for cause of failure, it is then cleaned and the components bead blasted to ensure that the components are clean and carbon free.
The parts are measured and checked for true and wear and the client is quoted on the repair or replacement of the unit should it be deemed to be uneconomical to be repaired. The client then has the option to have the turbo repaired or to take an exchange unit.