This thread is good timing, they just actually released an article on this very subject today. Let me see if I can link it...
1HZ ENGINES (Turbocharged) understanding the real story.
Before I start, this is also applicable to all turbo diesel engines.
We all agree that excessive combustion heat kills diesels (fact).
But what is the gauge of reliability when turbocharged? Is it,
Boost pressure?
AFR’s (Tuning)?
EGT’s?
Or is it the turbocharger causing it?
Answer, all of the above.
It seems everyone has formulated their own theories, but no one has actually put it all together yet. Understanding the big picture, whilst isolating the most common denominator with failures and or issues is very important. In this post we will summarise these contributing factors.
Boost pressure. Turbo boost pressure itself cannot harm a diesel engine in any way. Higher boost pressure usually means higher heated air, which makes the air less dense and easier for the cylinder to compress. But by raising the boost pressure too high on some turbochargers, it is inadvertently also increasing the turbine back pressure (pressure in the exhaust manifold). This back-pressure or drive pressure produces diminishing torque/power values due to some of this heated un-oxygenated gas being held back into the cylinder reducing the amount of oxygen the piston can draw in by occupying space in the cylinder. This excessive pressure also puts extra unwanted load back on the turbocharger bearing components. Accepted Industry standard for turbine back pressure, is a maximum limit of 1.4:1 (Exhaust Manifold Pressure to Intake Manifold Pressure Ratio). 1HZ engines are pre-combustion engines and have no cooling gallery in the piston crown from standard so are not well suited to removing the extra heat load generated. We feel instead of using boost pressure as the limiting factor, we suggest, progressively increasing the turbochargers shaft speed and tune until the maximum (safe) turbine back pressure is achieved is a much safer option.
AFR’s. We all know that excessive fuel can cause heat and fatigue diesel engines. But this alone as the governing factor for reliability is a false economy and I’m going to tell you why.
Turbine back pressure is a limiting factor for all diesels. It robs the engine of torque after peak boost pressure has been achieved, and raises the thermal load throughout the engine. This is significantly important to the 1HZ engine where the idea is to keep the cylinder temperatures cool to help engine reliable. When high turbine back pressure occurs, the engine’s VE (volumetric efficiency) drops due more very hot exhaust gas being held back in the cylinder. This extra spent gas occupying the cylinder from high turbine pressures, leaves less space for the in rush of cool intake air when the exhaust valve finally closes. The engine relies on this dense volume of intake air to keep the cylinder cool (before ignition) and to drive out the last residue of hot exhaust gas when the two valves are open by a pressure bias to the intake pressure being higher than the exhaust pressure. Testing has shown that the actual peak cylinder temperature (that is only for a split second) does not actually rise much, but the duration of this peak cylinder temperature under load is sustained for longer, resulting in a median temperature rise with more thermal transfer into the oil through the piston, into the water through the cylinder walls and the pre-combustion chamber. Which brings us to AFR’s and EGT’s.
A lower volume of intake air for the same fuel volume means the AFR’s will be lower and the EGT’s will be higher right – well that’s not really so. Yes, AFR’s will lower slightly and the EGT’s will also rise slightly. As the residual exhaust gas combined with pre-heating the intake air temperature, results in extra heat at the point the fuel is injected allowing more time for the fuel to burn, altering the AFR reading (as opposed to having a high oxygen content which will make the fuel burn faster). This results in high cylinder temperatures. For 30 degrees of extra EGT the cylinder temps under sustained load can rise by 380 degrees from its median, with the AFR only moving 0.6 points.
So - boost pressure, AFR’s and EGT’s are not the gauge for reliability on a 1HZ engine with the presence of high turbine back pressure. I haven’t bothered mentioning excessive pump timing or leaking injectors/ atomization etc as that is a given for diesel engine failure.
Some of the suggested possible solutions I’ve heard for running a turbocharger with high turbine pressure are,
“Run a much higher AFR to compensate.”
This goes against the very purpose of turbocharging in the first place which is to increase the engines air density in the cylinder, and is false economy. Pre-combustion 1HZ engines are designed to run lower AFR’s than a Direct Injection diesel (fact). Running a tune of 24/25:1 is a band aid fix and counterproductive.
“You should use an intercooler when turbo-charging any diesel engine.”
Intercoolers are great they add density or more oxygen to the same volume/pressure of air. This presents more oxygen in the same compressed space so the diesel fuel will ignite faster and more complete in the initial combustion cycle at the correct temp and pressure at just after TDC. We suggest fixing the turbine back pressure issue so you have less exhaust gas left in the cylinder causing late and in-complete ignition of the diesel, and making full use of the intercooler’s ability. A 1HZ engine with very low turbine back pressure can run a lower AFR safely as is the design of pre-combustion for its efficiency (with or without an intercooler).
Summary and conclusion.
Any turbocharger can be used with a 1HZ engine, some will be much better than others. It’s first and foremost limit for reliability is the turbine back-pressure, once this parameter is set and not an issue - tuning (AFRs and EGTs) can be implemented. I think you will find a lot of these “quick spooling” turbochargers will have very high turbine back-pressure and will be limited. After all, the whole idea is to burn fuel with the maximum amount air density available, and not restrict or reduce this air density in any way. Turbine back pressure is measured as a Ratio dividing the exhaust manifold pressure (EMP) by the intake manifold pressure (IMP). EMP 21psi, IMP 15Psi = 21/15 = 1.4:1 EMP: IMP. Testing of our exhaust manifolds showed the importance of a divided manifold for cylinder scavenging, by reducing the possibility of exhaust valve overlap that can also result in gasses being retained within the cylinder due to pulse energy disruption. Similar, but another subject.
Guys, before we get inundated with emails and calls – We don’t yet have a 1HZ turbocharger ready for the 1HZ engine and can only point you in the right direction. As you can appreciate it takes time to engineer things properly, but we felt it was best to share this insight so 1HZ owners and in fact all turbo diesel owners could benefit by preventing a few costly errors.
