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Turbo talk
Atmospheric pressure at sea level is 14.7 psi
To complete its cycle, an internal combustion engine needs to draw air into the cylinders. This is achieved when a piston lowers in its cylinder, causing a vacuum, allowing the higher ambient atmospheric pressure to push air into the cylinder ready for the next explosion.
Air contains oxygen, more oxygen makes better combustion. This has been understood since the late 1800s, when Daimler patented a gear driven air pump, or supercharger.
The turbocharger was patented by Sulzer Diesel engines in 1905. Air pressure pumps became very useful in piston engined aircraft, because the atmospheric pressure lowers as altitude increases. As the 'plane climbs the pressure difference lowers between the cylinder vacuum and the outside air, and the engine's efficiency lowers.
A supercharger puts a mechanical load on an engine, in the wartime Merlin engine its supercharger used 150hp to generate an extra 400hp. So a supercharger puts two strains on an engine, one driving the pump, and the other the extra combustion
forces generated.
Turbo charging puts no mechanical load on an engine, the cylinder's extra exhaust stroke turbo resistance pressure is balanced by the turbo's increased air pressure on the vacuum stroke pistons
1 Compressor Inlet
2 Compressor Discharge
3 Charge air cooler (CAC)
4 Intake Valve
5 Exhaust Valve
6 Turbine Inlet
7 Turbine Discharge
The spooling delay, or 'Lag', can be reduced by changing the turbo's aspect ratio, or gearing, by reducing internal friction, and improving the wastegate response. A bigger turbo will increaser power, smaller will increase response. The internal design of the turbocharger will be tailored for a particular engine's greatest benefit, optimising the engine's requirements.
A turbocharger's compressor supplies air to the inlet manifold at increased pressure, this fan is driven by the pressure of the escaping exhaust gas.
'Boost' is the amount that the manifold pressure is increased over the atmospheric pressure. This is expressed in bar, 1 bar being 15 psi.
Control of the boost pressure is required to keep the engine system within its designed operating range. This is controlled by the wastegate, which allows excess exhaust pressure to bypass the turbine impeller. Too high manifold pressure will cause internal damage to an engine.
In industrial engines, turbo lag isn't a problem, so a large turbo can be used to raise thermal efficiency. In a smaller car engine the turbo is used to increased power and acceleration, what's required here is a small, fast reacting turbo, this reaction time is known as 'Spooling'. Superchargers of course don't suffer spooling delays, being directly driven by the engine.
A Sulzer Diesel engine being shipped to Jersey to generate electricity
John Garrett founded his air compressor business in Los Angeles in 1936, to service the aero industry, the company is now part of the Honeywell group.
Our Lancia Deltas use a Garrett T3 turbo, the T series turbos were originally designed in the 1950s. These early generation turbos use older technology and are larger and heavier than modern units. However, the simple robust design means good reliability, and internal components can be improved and updated.
While under my Delta on other business, I removed the lower turbo hose and found half a cup of engine oil in it, this is a sure sign of a worn turbo, the seals and shaft were allowing oil into the inlet tract..
So I removed the item and took it to GP Turbos in Lancashire, Gary noted wear, and impeller damage, this damage is caused by small items entering the inlet tract. Gary gave me a quick talk,
and took the turbo in for repair.
Turbo going for a ride
Turbos at GP
I left the turbo with GP, and went home to read the Garrett website. It's not a 5 minute read! Visit the site here
The Garrett website is full of useful information, here's some more of it...
Careful turbo installation is important, the primary input for turbo selection is the target horsepower requirement, this is governed by the fuel and air flow, and the engine components' durability. Physical installation also requires planning, ball bearings require less oil than plain bearings, so in that case a restrictor to optimise oil pressure at 45-50psi is recommended, and the oil drain must be clear and short. The water feed must be adequate, and the drain designed to allow the self-syphoning process to function.
The turbo's water jacket surrounds the bearings area, and when the engine stops the exhaust component's heat soaks into the turbo, Garrett's design
allows self-syphoning to continue coolant circulation.
A turbo needs lots of oil to cool and lubricate, there must be a full flow designed into the system. The main source of internal turbo wear is carbon particles, this is caused by the high turbo temperatures acting on the mineral oil used, causing hard carbon particles to form. These particles rapidly wear the bearing surfaces, the remedy is to always use synthetic oil.
Manifold design on turbocharged applications is deceptively complex as there many factors to take into account and trade off General design tips for best overall performance are to:
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Maximize the radius of the bends that make up the exhaust primaries to maintain pulse energy
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Make the exhaust primaries equal length to balance exhaust reversion across all cylinders
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Avoid rapid area changes to maintain pulse energy to the turbine
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At the collector, introduce flow from all runners at a narrow angle to minimize "turning" of the flow in the collector
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For better boost response, minimize the exhaust volume between the exhaust ports and the turbine inlet
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For best power, tuned primary lengths can be used
Cast manifolds are commonly found on OEM applications, whereas welded tubular manifolds are found almost exclusively on aftermarket and race applications. Both manifold types have their advantages and disadvantages. Cast manifolds are generally very durable and are usually dedicated to one application. They require special tooling for the casting and machining of specific features on the manifold. This tooling can be expensive.
On the other hand, welded tubular manifolds can be custom-made for a specific application without special tooling requirements. The manufacturer typically cuts pre-bent steel U-bends into the desired geometry and then welds all of the components together. Welded tubular manifolds are a very effective solution. One item of note is durability of this design. Because of the welded joints, thinner wall sections, and reduced stiffness, these types of manifolds are often susceptible to cracking due to thermal expansion/contraction and vibration. Properly constructed tubular manifolds can last a long time, however. In addition, tubular manifolds can offer a substantial performance advantage over a log-type manifold.
Here's one part of one page referring to exhaust manifolds...
There's more.....
To the right we have..parts of the Compressor Map: The compressor map is a graph that describes a particular compressor’s performance characteristics, including efficiency, mass flow range, boost pressure capability, and turbo speed. Shown is a figure that identifies aspects of a typical compressor map:
I collected my refurbished unit, GP had fitted a new shaft and exhaust impeller, and balanced the air compressor fan.
The wastegate had been serviced and tightened, and tested for correct operation, the casing and components were all cleaned, and painted where required.
Gary recommended not fitting a gasket between the turbo flange and the exhaust manifold. He said the very high temperatures at this point can destabilise the stainless steel gasket, he's seen them be burst when contacted with a splash of water.
Gary advised metal to metal, no exhaust paste either, it won't take the heat.
Back to Garrett's site again, they say when installing, carefully check the oil feed gaskets are lined up, so there's no oil flow restriction. Before fitting the oil inlet pipe, fill the turbo bearing section with engine oil, and when filling the car's cooling system use the correct grade anti-freeze mixed
with de-ionised water.
Don't start the engine until the oil pressure has built up, and flow is sufficient to lubricate the turbo bearings. Garrett say a dry start up will destroy the turbo bearings immediately.
The turbo's energy source is the exhaust gas temperature and pressure, aluminium can't be used in the turbo, it would melt at the 1000oC exhaust temperature, so special alloys were developed.
The air ways and air filter cabinet must also be carefully cleaned before installing the turbo, any bits drawn into the turbine will damage the air compressor wheel. This can turn at over 250,000rpm, so even a small particle can cause damage,
like in outer space!
And again, my favourite...use synthetic oil!
An oldie but a goodie!
Any excuse to show a can of my favourite oil, it's really expensive, but perfect for a high performance turbo engine.