ENGINE PERFORMANCE FUNDAMENTALS AND JAGUARS’ ENGINES

Article by Martin Joyce

In my last article, I closed it with a comparison of the specific power of Jaguars’ petrol engines throw ugh the years and showed how performance has constantly increased and indeed how modern road car engines now have higher specific power than their racing forefathers of as little as 10 years earlier.

In this article I will explain the fundamentals of how engines make power (and indeed torque) and then how the increase in specific performance has been possible despite the simultaneous need to improve fuel economy, emissions, reliability, quietness and drivability (amongst others).

I will apologise in advance to any deeply versed in the art of tuning engines as out of necessity to both limit the article to around 2,500 words and to try to make it intelligible for lay-persons, I have simplified matters in a numbers of places. Equally I apologise to any lay-persons for whom it is too technical – I have done my best to simplify matters, but after 35 years on the subject I may be too deeply engrained!

Before going any further I want to clarify what is power and what is torque. Power

is direcly propotional to torque multipled by RPM and is defined by the equation:

Power [BHP] = Torque [lbsft] x RPM [revs/min] / 5252

So when someone says an engine “has good low speed torque” they could equally say “it has good low speed power” and the same is true at high higher RPM. It is just a convention that people talk about torque at low RPM and power at high RPM.

I will conform to this convention as it is the common parlance and also because changes are more apparent when talking of torque at low RPM as can be seen in Figure 2, a typical 1990’s torque curve, although one dear to my heart given my professional investment in AJ26.

Engines’ are basically air pumps into which fuel is added. When this air/fuel mixture

is ignited, the resulting increase in pressure pushes the piston down and by acting

on the lever arm of the crankshaft, via the conn rod, gas pressure is turned into torque.

So the power/torque of an engine is determined fundamentally by

a) How much mass of air you can induct

b) The level of motion in the air (squish/ swirl/tumble)*

c) The quality of mixing of fuel and air*

d) The optimising of ignition timing*

e) The internal losses due to friction between the piston and the flywheel

*Efficiency of combustion

The first, and most important of these is the mass of air into the engine. Engines are

volumetric devices. With each rotation of the crankshaft, the volume of the cylinder changes by an amount determined by the square of the bore x stroke x 3.142 / 4. However, the power of an engine depends on the mass of air, so the density of the air is also very important, both air temperature and air pressure, as it passes through the inlet valves.

Getting high density air into the engine can be achieved by a number of methods. Firstly, and in some ways most simply, a boosting device, supercharger or turbocharger, can be used to increase the pressure of the air past the valve. Unfortunately, the compression of air by these devices also heats the air, so in most cases an intercooler is used to cool the air back close to atmospheric temperature. On un-boosted engines (naturally aspirated or NA’s) there are two areas where the design of the engine affects this.

The first of these is “static” density resulting from pressure drops in the intake system,

cylinder head and exhaust system. As the flow of air through the engine increases, these normally increase in proportion to the square of air flow, so the effect of high losses is largest at high RPM and fairly small at low RPM. The second I have termed “dynamic” denisty. This is the tuning of the inlet & exhaust systems to achieve a high pressure at the inlet valve shortly before it closes in the case of inlet tuning and a vacuum at the exhaust valve just before it closes.

The strength of this effect can be complicated (and indeed is damped down by high pressure losses) but can be as high as 15~20% on highly tuned road engines and even higher on racing engines. This has also led to the use of “variable” features (inlet manifolds and cam timing) so the engine speed at which a high inlet pressure (or exhaust vacuum) occurs can be at multiple speeds and/or a broader speed range rather than a single speed (and harmonics of it) is the case in an engine with no variable features.

This is one of the areas where fuel injection has enhanced the performance of engines as in the past engines with carburators had convoluted intake manifolds, connecting one carb to multiple cylinders and with water heating of the manifold, in order to improve fuel atomisation for good drivability, but unfortunately being bad for power as they caused high pressure drops and heated the intake air.

In the past getting fuel into the cylinder was a challenge due to the limitations of carburators and the fact that in order to maintain a relatively constant air flow

through them they were normally shared between 2, 3 or 4 cylinders.

For racing engines this was not a problem as they used a carburator (or at least a choke) per cylinder, but this led to fuelling issues that made meeting emissions and drivability requirements very difficult and/or expensive to achieve. Since the invention of fuel injection getting enough fuel into engines has not been a significant issue but it introduced new issues in terms of atomising the fuel and mixing it well with the air. With port injection used since the 1970’s this was managed by careful targetting of the injector to spray onto the hot inlet valve. Since the adoption of direct injection (DI) with the injector inside the cylinder, this problem has grown as the time for injection and atomisation is very limited.

The fundamental solution to this has been to increase injection pressures, from ~60 psi for a port injected engine to ~1000 psi for the earliest DI engine and now ~5000 psi in the later engines launched from 2018 onwards. (To put this into context, there were diesel engines using ~3000 psi in 1916 and the latest diesel engines use injection pressures of 30,000~45,000 psi.)

This high pressure, coupled with carefully designed injection patterns and combined with extensive use of computer simulation of air and fuel flow in the cylinder by a method called Computational Fluid Dynamics (CFD), has resulted in modern GDI engines being cleaner and more powerful than any previous gasoline engines.

Having got as much air into the cylinder and added the fuel needed for maximum power (normally at 12~13:1 Air/Fuel Ratio), we now turn to the efficiency of combustion. This is probably the most complicated of topics in terms of engine tuning, as a huge number of variables in an engine can affect this. However, the most important ones are compression ratio, air temperature, fuel octane, and air motion (swirl/tumble). (You will notice air temperature has featured again – hot air is really bad for good performance!)

The reason these are important is that maximum combustion efficiency is achieved if the maximum cylinder pressure is achieved at about 13 Deg after Top Dead Centre (TDC). Ideally (from a power and efficiency point of view, although it would likely break the con rod/crankshaft), 100% of the chemical energy from the fuel would be released at this point, increasing the volume of gases in the cylinder, raising the cylinder pressure and pushing the piston down.

In reality, the air/fuel burns progressively and the pressure achieved depends on the rate of burn and how this corresponds to the piston position as it drops down the bore as the crank rotates.

This ideal pressure curve is rarely achieved because the fuel quality (octane) is inadequate for the compression ratio of the engine without detonation (or pinking) occurring. Detonation is the unwanted combustion of fuel in the cylinder due to the local pressure/temperature increasing to a level high enough for self-ignition.

It can be avoided by retarding the ignition so the cylinder pressure rise is lower and later in the engine cycle, such that the high pressure needed for auto-ignition does not occur. Obviously, the compression ratio could be reduced, but this also reduces combustion efficiency.

As a result, the best compression ratio is selected to balance the loss of efficiency from a low compression ratio and the loss of efficiency from retarding the spark to avoid detonation.

In an attempt to avoid this undesirable trade-off, engine designers try to reduce detonation by other measures, including faster combustion from high tumble/swirl, good air/fuel mixing, reduced air temperature from good intake design, plastic inlet manifolds (insulating), and with boosted engines, the use of an intercooler.

The final part of the performance fundamentals is to minimise engine friction as much as possible. I am going discuss friction in more detail in a later article on fuel economy as it has an even bigger impact on economy, but suffice to say that at 5500 RPM the AJ16 engine lost around 50 BHP to friction, whereas the AJ26 lost approximately 40 BHP (both only the mechanical friction, excluding intake and exhaust pressure losses).

The work on friction reduction, aimed mainly at fuel economy improvement, actually gave 15~20% of the improvement in power that was achieved from the AJ16 to the AJ26 (despite the increase in cylinders from 6 to 8 and the addition of a 2nd cylinder head!), so the effect of friction on engine power is not negligible.

So, having worked through the fundamentals of engine power, I will return to how, despite the constant need to reduce emissions and improve fuel economy, the specific performance of Jaguar’s engines has increased with each generation.

The answer is, of course, primarily through the use of improved technologies but also, and especially since the 1990s, through the use of computer simulation to enhance the effectiveness of the use of those technologies and also from the computing power of modern Engine Controllers to accurately control some of the complicated technologies.

When launched in 1949, the XK was an engine designed to deliver performance and specifically power – hemi-spherical combustion chamber, double overhead cams (DOHC), and triple carbs coupled with a tuned exhaust and minimal intake and exhaust silencing were the “key technologies” of the 1950s.

In lower power versions, twin carbs and more moderate camshafts were coupled with silenced intake and exhaust for the drivability and refinement suitable for saloon cars and, increasingly over time, an emphasis on emissions reduction.

Later in the XK’s life, electronic fuel injection was added, but not really for power, the focus being on low speed torque and emissions reduction.

The XK was Jaguar’s first “in-house” engine and allowed them to break the 200 BHP barrier.

The V12 engine that followed was, in its road-going 5.3L version (unlike the prototype 5.0L intended for racing), a much more “rounded” proposition at launch, with a balance between low and high speed RPM and less emphasis on ultimate power potential.

The Single Overhead Cam (SOHC) was chosen for compactness (to fit in the car), lower cost and lightness, but could still deliver decent power with good low-speed torque.

Like the XK, the V12 initially was equipped with carburettors (4 off), but soon in its life it moved to fuel injection and gained ~20 BHP from a well-designed tuned inlet manifold which the injection system permitted.

Interestingly, this manifold lived on to the end of the V12’s life with only minimal modifications, so it was clearly well designed originally.

The V12 did not feature any particularly special technology for performance, relying on its large cylinder count and relatively large displacement for its appeal.

(Not to forget, of course, that in racing form at Le Mans the engine capacity was increased and displaced up to 7.4L, producing well in excess of 700 BHP, so there was plenty of potential in the design that wasn’t fully exploited in the road applications.)

The V12 was the first road-going Jaguar engine to exceed 300 BHP.

The AJ6 that followed introduced a key technology to Jaguar engines: 4 valves per cylinder, which, now almost universal, was a rarity when first used on the AJ6.

Specific performance (peak power and peak torque) took a significant step, allowing the engine displacement to be reduced by 14% compared to the XK, with a resulting fuel economy advantage (the first “down-sizing” technology).

The engine featured another well-tuned intake manifold and also an exhaust with a level of tuning for low-speed torque.

The AJ16 took the foundations of the AJ6 and enhanced its performance in two ways. The NA engines received new cams, ports, and intake manifold which, coupled with a freer-flowing exhaust, lifted max power by around 10%.

Meanwhile, in a bid to compete with the BMW M Series cars, the Supercharger AJ16 was created, starting over 2 decades of Supercharged Jaguar engines and arguably starting the “horsepower war” with the German OEMs that sees output of over 600 BHP now being common.

The AJ16 is also notable for being the first Jaguar engine launched where catalysts were standard and the performance for the USA market was the same as for the UK. Until then, the performance in the USA was always less due to catalysts, low RON fuel, and other emissions needs, typically 10~20% worse in the USA.

The AJ26 saw a number of new technologies; significantly lower friction, a plastic intake manifold on the NA, a large valve area and inlet cam Variable Valve Timing (VVT) on the NA, and a larger supercharger (SC) on the SC engine saw the AJ26 produce 15~20% more power than the AJ16 in both NA and SC forms.

Figure 8 - Technology of Jaguar’s engines through the generations

The later 4.2L version of the SC V8 engine being Jaguar’s first to exceed 400 BHP.

The AJ26 evolved into the 5.0L AJ133 V8 and performance jumped again, first through the 500 BHP level at launch and later 600 BHP in SVO form in its later SC versions.

The engine used a whole range of upgrades from AJ26; VVT on both cams, direct injection (~2200 psi initially), low friction, optimised intake and exhaust systems, Variable Valve Lift (VVL) on the NA, and a still larger and more efficient TVS supercharger on the SC engine.

And finally, the Ingenium engine family, the latest and most high-tech engine from Jaguar.

Specific performance is now up to 150 PS per litre.

Key technologies on the Ingenium build on those of the AJ133; low friction, direct injection, dual VVT, VVL, and add turbocharging, with the 2.0L 300 PS version featuring a TVS Supercharger and Turbocharger working in tandem.

The very latest 3.0L (yet to feature in a Jaguar) pairs an electric supercharger with a twin-scroll turbocharger.

Turbochargers are able to generate higher boost levels than Roots superchargers, which increases air density and permits higher specific performance, but normally results in “laggy” performance unless augmented with a supercharger of some sort, as has been done with the Ingenium engines.

In a future article I will go into the differences between turbochargers and superchargers in detail and explain why these phenomena exist.

This interestingly emphasises a common theme through almost every Jaguar engine; the importance of a balanced torque curve with good low RPM torque coupled with adequate power to meet the market’s needs for power, which have grown decade after decade.

In the early days this was achieved through the appropriate selection of bore, stroke, cam profile, and inlet/exhaust tuning.

Now, in the 21st century, whilst ensuring the engine fundamentals are in place, the selection of the boosting technology is key to delivering the character of performance desired, with supercharging having provided one step in performance and then supercharging and turbocharging together providing a 2nd step of a similar size again.

So, what does the future hold?

More challenges, and primarily how to continue to deliver the desired, and increasing, performance always wanted whilst meeting the constantly growing emissions and fuel economy/CO2 demands of governments.

Can “the circle be squared”?

I have no doubt it will be, and that we can expect to see still more complex engines which will be coupled, in many cases, with levels of hybridisation to meet the ongoing regulations and market needs.

In the next articles I will talk about Fuel Economy and Emissions, the two now ever-constant pressures for the improvement of the Internal Combustion Engine.

These challenge and inspire engineers to innovate and optimise and to continue to develop the engine to a level many thought infeasible only 20 years ago.

Story taken from Xclusively Jaguar Magazine - October 2020 issue.

Read the full issue here.